91Ó°ÊÓ

To add two numbers with similar signs, add their absolute values and keep the similar sign with the resulting number.

To add two numbers with opposite signs, subtract the absolute value of smaller number from absolute value of larger number and keep the sign of larger number with the resulting number.

The difference of the numbers $7-3=4$is positive because the larger number is positive as well.

And the difference of the numbers $3-7=-4$ is negative because the larger number is negative as well.

To evaluate the sum $-28+\left(-40\right)$,observe that both the numbers have similar sign. So, the numbers will add and the resulting number will carry the same sign, that is, negative.

So, the sum is

$\begin{array}{c}−28+(−40)=−28−40\\ =−68\end{array}$

Thus, the sum of the given numbers is $-68$.