Angles B and C are both supplimentary angles with the numbers they are closest to. The sum of two supplimentary angles is always 180 degree. Therefore:
B + 45 = 180
B = 135
C + 105 = 180
C = 75
We now know two out of three of the angles for the inside of the triangle. The sum of the interior angles of a triangle is always 180. Therefore, we can solve for A.
A + 75 + 45 = 180
A = 60
Therefore A = 60, B = 135 and C = 75.