Perimeter<span> and area are two important and fundamental mathematical questions</span>
I'm assuming you mean triangle angles?
If so, all angles add up to 180 so combine them and set them equal to 180.
51+82+5x+2=180
135+5x=180
5x=45
x=9
(x-5)(x+2)=0
roots: x=5 and x= -2
see attached photo for steps