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Algebra is a branch of mathematics that substitutes numbers with symbols like letters to represent quantities in equations and formulas. By using letters as variables, we can create equations that describe relationships between different values. This flexibility allows us to solve problems involving unknowns. In school, algebra often starts with linear equations—these are equations where variables have a power of one, often looking like simple combinations of the form \( ax + b = c \).
For students just starting with algebra, the key is to understand that the objective is often to find the value of these unknowns. In our exercise, we are tasked with finding the value of \( D \) in the equation \( 5D - 2 = 13 \). Keeping this objective in mind can help guide every step you take in solving the equation.

Isolating variables is a fundamental step in solving equations in algebra. The whole idea is to get the unknown variable by itself on one side of the equation. This helps us determine its value. To achieve this, you can use a variety of algebraic operations such as addition, subtraction, multiplication, or division.
In the exercise provided, to isolate \( D \), we wanted to eliminate the constant term on its side of the equation. Hence, we added 2 to both sides, neutralizing the \(-2\) on the left side. Remember, whatever you do to one side of the equation, do to the other to maintain balance. After this step, the equation becomes \( 5D = 15 \), a much simpler form that is ready for the final step of solving.

Simplification is a process that makes equations easier to solve by reducing complexity. After isolating the variable term, our goal is to break it down until the variable stands alone. In our example, once \( 5D = 15 \) was achieved, the next step was to simplify further by dividing both sides by 5. Why divide by 5? Because it's the coefficient of \( D \)—the factor that’s multiplying \( D \) in this equation.
By dividing, we cancel out the coefficient on the left, simplifying our equation to \( D = 3 \). This process is crucial because it enables us to find the direct value of the variable, resolving the equation entirely. By practicing simplification, you not only find the solution but also develop a deeper understanding of how different parts of an equation affect each other.