0=4x^2-21

Solution for 0=4x^2-21 equation:

Simplifying
0 = 4x2 + -21

Reorder the terms:
0 = -21 + 4x2

Solving
0 = -21 + 4x2

Solving for variable 'x'.

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

Add '-4x2' to each side of the equation.
0 + -4x2 = -21 + 4x2 + -4x2
Remove the zero:
-4x2 = -21 + 4x2 + -4x2

Combine like terms: 4x2 + -4x2 = 0
-4x2 = -21 + 0
-4x2 = -21

Divide each side by '-4'.
x2 = 5.25

Simplifying
x2 = 5.25

Take the square root of each side:
x = {-2.291287847, 2.291287847}