Who can help me solve this differentiation exercise?

a) A simple pendulum consists of a small metal ball attached to a fixed point by a cord of length l centimetres. It takes T seconds to make one swing, where T=0.2sqrtl. Find dT/dl. (This I have solved) b) The cord originally has length 2.5 metres. If it is lengthened by 2 millimetres, calculate what effect this will have on the time of swing: I) by using your answer to part a. II) by direct evaluation of T.