I know the vertex formula, which shows the highest point, is -b/2a. My function is f(x) = -.118x^2 + 2.11x + 4.215

When I use this for the vertex, I get -2.11/ 2(-.118) or 2.11/.236 which is 8.94, but the correct answer is 13.647.

This 13.647 also makes sense when I look at the table of distance and height, but I'm not sure what I am doing wrong in my calculations.
Thank you.