Hint:
tp = a+(p-1)d = q ..................(1)
tq = a+(q-1)d = p ..................(2)
tp+q = a+(p+q-1)d ..................(3)
There are two unknowns in (1) and (2) and there are two equations and hence one can solve for them.
After solving, there values can be put in (3) and the result can be obtained.