It is given that sum of n terms of AP is nP + 1/2 n(n-1)Q
To find : the common difference
Substituting n = 1 gives the sum of the first term, that is the first term itself
a1 = P
Substituting n = 2 gives the sum of first two terms
a1 + a2 = 2P + Q
a2 = P + Q
Now common difference d = a2 - a1 = Q