fist put it in the triangle area formula: A = b * 1/2h
plug in your numbers:
120 = b * 15 or bc you didnt say if it was divided by 1/2 or not i will do both so it could be this too: 120 = b * 7.5 since 7.5 half of 15
solve:
120= b*15
b=8
OR
120=b*7.5
b=16
so 8 or 16 is your answer
depends if 15 was already in half or wasnt