I started this problem, but got stuck shortly after beginning
Basically, I started with the middle peice of math from the in/equation above.
Since v is a constant vector, I took it out the front of the integral and used the fundamental theorem of calculus such that the integral of the derivative collapses to γ(t) with the terminals b and a.
Subbing in the bounds, leads to γ(b) - γ(a), and given the equations in the equation, this simplies to v[Q-P], notice how this is different to the left hand side of the in/equality (PQ)v.
Is my method on the right track, or have I gone completely off the rails?