Solve x=-2y^2+16y?

Solution of an algebraic problem with 2 variables such as 'x' and 'y' need 2 equations. (Ans.)

x = -2y (y - 8)

Roots are (0, 0) and (0, 8)
Min (32, 4)
Parabola opening to the left

Are you trying to solve for y in terms of x?
x = -2y² + 16y
-1/2 x = y² − 8y
-1/2 x + 16 = y² − 8y + 16
16 − 1/2 x = (y − 4)²
±√(16 − 1/2 x) = y − 4
y = 4 ± √(16 − 1/2 x)

"Solve" has a multitude of meanings. WHAT do you want to "solve" exactly? WE are not mind-readers. Be specific.