It's just normal integration. Note that there's a 2/a outside one bracket and a 1/2 immediately to its right. So multiplying them (during the integration step) gives 1/a. That's not affected by integration because x is the integration variable (not a).
Regarding the a not cancelling: check your very last step.
General tip, either use decimals or (better) only fractions. Something like '2*pi*(0.25a)/a' is unnecessarily complicated notation. (2*pi*a)/(4*a) is so much easier to clean up in a single step without a mistake. It's also faster to write in an exam.