Mark M. answered 02/08/21
Retired math prof. Very extensive Precalculus tutoring experience.
sina = 7/25 and angle a is acute, so cosa = √(1 - sin2a) = 24/25 and tana = sina/cosa = 7/24
cosB = 12/13 and angle B is acute, so sinB = √(1 - cos2B) = 5/13 and tanB = sinB/cosB = 5/12
Tan(a + B) = [Tana + TanB] / (1 - TanaTanB) = (7/24 + 5/12) / (1 - 35/288)
Multiply the top and bottom by 288 to get (84 + 120) / (288 - 35) = 204 / 253