SATMathematics Practice Test 1

Subtracting the left and right sides of $\mathit{x}+\mathit{y}=-9$ from the corresponding sides of $\mathit{x}+2\mathit{y}=-25$ gives $\left(\mathit{x}+2\mathit{y}\right)-\left(\mathit{x}+\mathit{y}\right)=-25-\left(-9\right)$,
which is equivalent to y = −16.
Substituting −16 for y in $\mathit{x}+\mathit{y}=-9$ gives$\mathit{x}+\left(-16\right)=-9$, which is equivalent to $\mathit{x}=-9-\left(-16\right)=7.$