91Ó°ÊÓ

Two numbers with similar signs get added and the resulting number carries the similar sign.

Two numbers with opposite signs get subtracted and the resulting number carries the sign of the larger number.

If two positive numbers 2 and 3 are being added, then the resulting sum will be $2+3=5$, which is positive as well.

And if two negative numbers $-2$ and $-3$ are being added, then the resulting sum will be $\left(-2\right)+\left(-3\right)=-5$, which is negative as well.

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 expression $15-x$, substitute the value of x as 6 and add the numbers as shown below,

$\begin{array}{l}15−x\\ =15−6\\ =9\end{array}$

Thus, the value of the expression is $9$.