Can someone pls pls help me?

Which best explains why this triangle is or is not a right triangle?

Simplify. −4x^2 (5x^4−3x^2+x−2) −20x^6−12x^4+8x^3−8x^2 −20x^6+12x^4−4x^3+8x^2 −20x^8+12x^4−4x^2+8x −20x^6+12x^4+4x^3−8x^2

Solve for x and y: 217x + 131y = 913; 131x + 217y = 827