How to calculate cumulative probability distribution when the distribution is Trapezoid

Question

recently I am doing this question

I want to find the waiting time P(2>X>7).

I try to do this with:

P(X>2) - P(7>X>10) =(3+8) * f(2) / 2 - 3 * f(7) / 2

I can get f(2) = 0.13333

However, how can I get f(7)? Thank you