235=0.05v^2+2.2v. What does v equal?

Question

The distance d (in feet) it takes to stop a car traveling at v miles per hour can be modeled by d= 0.05v^2+ 2.2v. If the distance is 225. how fast was the car going when the brakes were applied 

Answer 1

225=0.05v²+2.2v,

0.05v²+2.2v-225=0,

v=(-2.2±√(2.2²+4×0.05×225))/(2×0.05) (the quadratic formula),

v=(-2.2+√(4.84+45))/0.1=48.6mph. (We reject the negative answer because it would imply a negative speed.)