Since is on the right side of the equation, switch the sides so it is on the left side of the equation.

Add and .

Subtract from .

Subtract from both sides of the equation.

Subtract from .

Multiply by .

Add to both sides of the equation.

Divide each term in by .

Cancel the common factor of .

Cancel the common factor.

Divide by .

Solve for a an=an-1+an-2