## Precalculus (6th Edition) Blitzer

$−3$.
When using the addition method, we need to have the opposite value of the coefficients before the variable. Thus, after we make the coefficient to be $-6$ before $y$ in the first equation, we need to multiply the second equation by a factor of $−3$, so that the coefficient of y becomes $6$ such that the variable y will be cancelled when adding the two equations. In other words, the answer is $−3$.