You would use sin to get the answer. So it'd be sin(72) = x/12
Once you get the sin you would multiply it by the 12 to get the x by itself and that's your answer.
It would be sin because the question is asking for that side, and it's across the 72, making it the opposite side, the side that is across from the 90 degrees is always the hypotenuse and you have a number for that side and sin uses the opposite and hypotenuse
