What do you know, another guest that is trying to get past his\her unit homework....but this does not make sense...isn't it summer break? Anyways, you asked for help, so here are the things you must do to accept the help in return.
1. CLOSELY follow my steps
2. Solve the bottom yourself. (answer by replying to my answer)
If you have done these steps, congrats, you are not a cheat. If you have not, then it is either that you have not seen my answer yet, or you just want the answer and leave without learning.
Anyways, the answer starts here:
Assuming you are in precalculus (im not sure how to help you if you are not), we can use the Law of Cosines
This is where c is opposite from angle C, a is opposite from angle A, and so on. USE THIS ONLY IF YOU HAVE MORE SIDES THAN ANGLES. IF YOU HAVE MORE ANGLES THAN SIDES, USE LAW OF SINES.
Anyways, just because the length we are looking for is called a, does not mean that it is a. It would be a whole lot easier if the line was "c", as the angle would correspond, so let that just be true.
According to the law of cosines, our equation is now:
c2 = 122 + 152 - 2(12)(15)(cos(110))
c2 = 144 + 225 - 360(cos(110))
c2 = 369 - 360(cos(110))
Sadly, cos(110) is not a nice number, but since this is a question, we should just round, unless you can tell me below whether rounding is or is not aloud, same with calculators.
c = sqrt(369 - 360(cos(110)))
c ~ 22.183941... ~ 22
Therefore, c is around 22