"how did u get 5 and do i but the original equation in y = ax^2 + bx+c "

YOU said that the function was y= **5**x^2+ 8x- 5. Since the leading coefficient is positive this is a parabola that opens upward. It has no "maximum value".

You could also see this by completing the square: y= 5(x^2+ (8/5)x+ 16/25- 16/25)- 5= 5(x+ 4/5)^2- 16/5- 25/5= 5(x+ 4/5)^2- 41/25.

When x= -4/5, the squared term will be 0 and y(-4/5)= -41/25. Since a square is never negative, y is never less than -41/25 so that is the **minimum** value. There is no bound on how large y can be and so no maximum.