## Precalculus (6th Edition) Blitzer

$-2$.
When using the addition method, we need to have the opposite value of the coefficients before the variable. Thus, we need to multiply the second equation by a factor of $-2$, so that the coefficient of $y$ becomes $-10$ such that the variable $y$ will be cancelled when adding the two equations. In other words, the answer is $-2$.