## Algebra 1

Given that the sum of the angle measures of a quadrilateral is 360 degrees, we can add all the angles to find x. (5x) + (4x + 10) + (80) + (90) = 360 (The 90 comes from the right angle, for a right angle is always 90 degrees). Next, combine like terms and simplify: (5x + 4x) + (10 + 80 + 90) = 360 9x + 180 = 360 Then, subtract 180 from both sides of the equation. 9x + 180 - 180 = 360 - 180 9x = 180 Finally, divide both sides by 9 to get x. $\frac{9x}{9}$ = $\frac{180}{9}$ The 9's on the left side cancel out, so x = $\frac{180}{9}$ Simplify. 180 $\div$ 9 = 20 x = 20