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%.