Math question yet again?

Question

There's this triangle where using the cosine rule. one side squared[times itself] can be found by y multiplied by y+2-2(y multiplied by y +2] multiplied by cosine 120. Then it says that the side is 7, and I have to show that y sqaured+2y-15=0. HOw do I do that?

Answer 1

use this
y(y+2)-2[y(y+2)]=7

factor and solve

for the next, use the opposite of FOIL for y^2 +2y-15=0
That is factor it.