Chapter 4 Quadratic Functions and Factoring - 4.10 Write Quadratic Functions and Models - 4.10 Exercises - Problem Solving - Page 314: 47

$f(x)=-0.0375(x-20)^2+15$.

If the vertex of a graph is at (m,n), then the general formula for the quadratic function is $f(x)=a(x-m)^2+n$. The vertex of the graph is at (20,15), hence the quadratic function becomes $f(x)=a(x-20)^2+15$. The point (0,0) is on the graph, hence if we plug in the values we get 0=400a+15. a=-0.0375, hence $f(x)=-0.0375(x-20)^2+15$.