I could be wrong, but shouldn't the graph of T^2 against l be a straight line for a pendulum?
Part 1 seems simple enough, draw a tangent at the minimum and root it.
I don't understand what you mean by part 2. The minimum distance (h?) is surely the value of h closest to the y axis?
Part 3 I imagine is either your answer to part two +/- either the graph scale or the graph scale plus precision of instrument.