Subtract from both sides of the equation.

Use the quadratic formula to find the solutions.

Substitute the values , , and into the quadratic formula and solve for .

Simplify the numerator.

Raise to the power of .

Multiply by .

Multiply by .

Add and .

Rewrite as .

Pull terms out from under the radical, assuming positive real numbers.

Multiply by .

Simplify .

The final answer is the combination of both solutions.

Solve Using the Quadratic Formula x^2-21=4x