We are told that the average speed of the vehicles can be modeled according to the curvature of the road. If the road is straight (that is curvature = zero) the average speed is: 46.26 mi/h. Each degree of curvature reduces this speed by 0.746 mi/h.
So we can write the following function S(D) (speed in terms of degrees of curvature) as
S (D) = 46.26 - 0.746 D
And therefore, for a curvature of 21 degrees we can evaluate it for D = 21 as:
S (21) = 46.26 - 0.746 (21) = 30.594 mph
which rounded to two decimal places is: 30.59 mph