use law of cosines to find an angle:
c^2 = a^2 + b^2 - 2*a*b*cos(gamma)
13^2 = 10^2 + 15^2 - 2*10*15*cos(gamma)
0,.52 = cos(gamma)
58.67 = gamma
use law of sines to find another angle after finding gamma:
sin(gamma) = sin(beta)
---------------- -----------
c b
sin(58.67) = sin(beta)
-------------- ------------
13 15
0.85 * 15 = sin(beta)*13
sin(beta) = 0.98
beta = 78.52
alpha = 180 - 78.52 - 58.67 = 44.81