Step-by-step explanation:
Your question seems to be "how do you solve this?" The answer is ...
make use of the relationships between sides and trig functions for angles in a right triangle.
___
The useful relationships here are ...
Sin = Opposite/Hypotenuse . . . . sin(A) = 9/15 = 3/5; sin(B) = 12/15 = 4/5
Cos = Adjacent/Hypotenuse . . . . cos(A) = 12/15 = 4/5; cos(B) = 9/15 = 3/5
You will notice that sin(A) = cos(B) and sin(B) = cos(A).
The relationship for tangent is ...
Tan = Opposite/Adjacent . . . . tan(A) = 9/12 = 3/4; tan(B) = 12/9 = 4/3
You will notice that tan(A) = 1/tan(B) and vice versa.
___
These relationships are summed up in the mnemonic SOH CAH TOA. It may help to think of this as a name in some language.