Firstly you haven't completed the integration - that is done with respect to the variable "t". Do that bit first. I note that you have dropped the 'dt' term in the final line - perhaps that is where you are getting confused.
You'll first have to square the terms within the square braces and integrate the (binomial) result. There will be resulting terms in t, e^(-t/τ) and e^(-2t/τ) - where τ is tau.
Having completed the integration you would then substitute the limits of integration [0,0.00835] into the integrated function where the limits are t=0 and t=0.00835.
You seem to be missing something important in your understanding of the integration process.