If x - y = 3 and x^2 - y^2 = 63, then find x^2 + y^2.
x^2-y^2 is the difference of squares: (x+y)(x-y)
So (x+y)(x-y)=63
We already know x-y=3.
3(x+y)=63
x+y=21
We can form a system of equations from this.
x-y=3
We solve and find that x=12 and y=9.
So x^2+y^2=12^2+9^2=144+81=225.
+781 
