Answer:
x = ±√(81 -y^2)
Step-by-step explanation:
Subtract y^2 and take the square root.
x^2 +y^2 -y^2 = 81 -y^2 . . . . . subtract y^2
x^2 = 81 -y^2 . . . . . simplify
x = ±√(81 -y^2) . . . . take the square root
