The crossover rate is he discount rate at which the NPV of both contracts is the same.

Unless you feel like solving a cubic, you can use trial and error to find the crossover rate.

Alternatively (and equivalently) you could find the IRR of the following Hypothetical Project:

(Project Q) - (Project P) = 7000,-1000,-2800,-5150

When i did it i got 10.6%.