“The breaking distance is directly proportional to the square of its speed” Using your variables, this means: d = k•b^2 k here is some unknown (and for this question irrelevant) number.
Now if b (the speed) is increased by 200%, then your new speed is b+2b = 3b. The 2b is the 200% increase.
Throw 3b into the first equation we have the new d as: d = k • (3b)^2
Simplifying this, you have d = k•9b or d=9kb
This means the new braking distance is 9 times the original braking distance.
