I'm doing a question for a math assignment and I have to find the width of a parabola using the equation h(d)=-0.4d^2-5d+8.75 I keep getting wrong answer

For us to help you please show us your work and why you think you have the wrong answer. And please define what you mean by finding the "width" of the parabola.

EDIT - I suspect that the term "width" means the value 4p in the standard parabolic formula:

4p(y-b) = (x-a)^2

where (a,b) is the coordinates of the vertex of the parabola and 'p' is the distance from the focus to the vertex (and also from the vertex to the directrix). Assuming this is wehat is meant - you need to restate the equation you were given into that form, using 'd' in place of 'x' and 'h' in place of 'y'.