81=21x^2

Solution for 81=21x^2 equation:

Simplifying
81 = 21x2

Solving
81 = 21x2

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Add '-21x2' to each side of the equation.
81 + -21x2 = 21x2 + -21x2

Combine like terms: 21x2 + -21x2 = 0
81 + -21x2 = 0

Add '-81' to each side of the equation.
81 + -81 + -21x2 = 0 + -81

Combine like terms: 81 + -81 = 0
0 + -21x2 = 0 + -81
-21x2 = 0 + -81

Combine like terms: 0 + -81 = -81
-21x2 = -81

Divide each side by '-21'.
x2 = 3.857142857

Simplifying
x2 = 3.857142857

Take the square root of each side:
x = {-1.963961012, 1.963961012}