If 3x + 5y = 4 and 8y + 6x = 36, what is the value of -y?

$- y = 14$

Explanation:

$3 x + 5 y = 4$
$8 y + 6 x = 36$
-y=?
$3 x + 5 y = 4 - - - - - - - - - - - - \left(1\right)$
$8 y + 6 x = 36 - - - - - - - - - - - - \left(2\right)$
$\left(2\right)$ can be simplified as
$3 x + 4 y = 18 - - - - - - - - - - - \left(3\right)$
Subtracting (1) from (3)
$4 y - 5 y = 18 - 5$
$- y = 14$