Answer
8y$^4$+2y$^2$+20
Work Step by Step
Distribute the negative sign to the parentheses and then combine like terms vertically.
$\qquad3y^4-4y^2+7$
$\underline{-(-5y^4-6y^2-13)}$
$\qquad\qquad\downarrow$
$\quad3y^4-4y^2+7$
$\underline{+\,5y^4+6y^2+13}$
$\quad8y^4+2y^2+20$