A Perimeter Problem

"A rectangle is 3 cm longer than it is wide, and its perimeter is 26 cm. How wide is it?" Call the width $x$. Then the length is $x+3$. The perimeter is the total of all four sides, which is two lengths plus two widths:

$$2\left(x+3\right)+2x = 26$$

Distribute to get $2x+6+2x = 26$. Collect the like terms to get $4x+6 = 26$. Subtract 6 to get $4x = 20$, then divide by 4 to get $x = 5$. The width is 5 cm and the length is 8 cm. Check that $8+5+8+5 = 26$.

In a geometry problem, a formula gives you the equation. The formula might be for the perimeter, for the area, or for the angles of a triangle adding to 180°. Write the formula with your expressions in it. The result is an ordinary equation, and you solve it in the usual way.