The point where 2 lines intersect contain (x,y) coordinates that are equal for both equations. By setting the 2 equations equal to each other, it is same as equating the 2 y-values, which we know they are equal at point of intersection.
this is because you have 3 angles so its a triangle you have 360 degrees so 360 - 108 = 252 you need to divide it by 2 as there are 3 angles so you get 126 i hope it helps