For starters, can you set up two equations from the two first two bits of information?
That is right, The formula for the sum of n terms of an AP is Sn= n/2 [ 2a + ( n-1) d ] Note that it should be 2a and not a.
So there is nothing left for now you have
10 [ 2a+19 d ] = p and 11 [ 2a + 21 d ] = q. now find a the first term and d the common difference and get the result.