The only way I know how to do this is to use the cosine law.
b^2 = a^2 + c^2 - 2*a*c*Cos(105)
a = 12
c = 10
b = ??? (AC)
Cos(105) = - 0.25882
b^2 = 12^2 + 10^2- 2*10*12*(-0.25882)
b^2 = 144 + 100 + 240 * (0.25882)
b^2 = 244 + 62.117
b^2 = 306.12 Now take the square root
b = 17.50