Let r1 be the 1 year spot rate, r3 be the 3 year spot rate, r1,3 be the forward rate. Then:

(1+r1)*(1+r1,3)^2 = (1+r3)^3

r1,3 = ((1 + r3)^3/(1+r1))^(1/2) - 1.

Plug in r1 = .12, r3 = .13 to get the answer of: .135. 13.5% is the two-year forward rate.