Again, form your equations.
This one requires a bit more prior thought
Equation1 is OK
T=(100/2)(2a+99d)
For equation 2, the important thing to grasp is that the common difference will be 2d, since we are summing every other term
[compare 2,4,6,8,10,12,14 here d=2
if you take every other term, 2.6.10... d=4]
So equation 2 is
(T/2)-1000 = (50/2)(2a+(49*2d))
When you rearrange the equations, you should get to 2 equations, both of which contain T and 100a, so if you subtract them, you can work out d, which is, fortunately, all you are asked for.
Aitch