Well you already know your side lengths, so you use this formula S=1/2(A+B+C). For A put in one of your side lengths, for B put one of your side lengths, and for C put one of your side lengths. After you do that solve. But your not done. Once you find out what S equals by doing the formula I just showed you, you have to do another formula which is A= S(S-A)(S-B)(S-C). Once again plug in your side lengths for A,B, and C but then plug in your S value. (Remember you already found your S value by doing the first formula I showed you). Then do the math and then you will find your answer.