The force of interest is : δ(t) = 0.05 + 0.001t + 0.0001tē

We were asked to calculate the total at time 10 of the accumulate proceeds of an investment of R100 at time 0 plus an investment of R100 at time 5.

This I correctly calculated to be R316.45.

However now it asks for the equivalent constant force of interest earned on the transaction. The answer is 5.97%... Please can somebody help me or give me some tips on how to get to this answer...