The non-negative real numbers x, y, and z satisfy the two equations
x^2 + y^2 + z^2 = 9
xy + xz + yz = 18
What is the sum of x, y, and z?
Use the following algebraic identity:
(x + y + z)^2 = x^2 + y^2 + z^2 + 2(xy + xz + yz) = 9 + 2(18) = 45
+1224 
