There you go!!! Hope I helped :)
Let the legs of the triangle be a and b, and the hypotenuse c.
Your first instinct might tell you to use the Pythagorean theorem to go about solving this because

. This works, but it is slow.
The fastest way to solve this is to recognize that the right triangle is a special triangle where the ratio of the sides are 3:4:5. This means that if the legs are 9 and 12, then the hypotenuse is 15 because 3*3 is 9, 3*4 is 12, and 3*5 is 15.
-20=-4x-6x
4x+6x=20
10x=20
x=20:10
x=2
proof:
-20=-4×2-6×2
-20=-8-12
(-8)+(-12)=-20
Well, the amount of degrees is 1,980, so plug that in so that the equation is 1980 = (n-2)180. Then, do order of operations. Divide 1980/180, which is 11. Then, just add 2. So, the polygon has 13 sides.