Evaluate using the formula .

Subtract from .

Rewrite as .

Cancel the common factor of .

Cancel the common factor.

Rewrite the expression.

Simplify the numerator.

Multiply by .

Multiply by .

Multiply by .

Simplify the denominator.

Expand to .

Simplify.

Multiply by .

Multiply by .

Multiply by .

Divide by .

Find the Number of Possibilities 18 choose 4