Answer
$x = 15$
Work Step by Step
According to theorem 6-19, the base angles of an isosceles trapezoid are congruent.
Let's set the two base angles, $\angle B$ and $\angle C$, equal to one another to find the value of $x$:
$3x + 15 = 60$
Subtract $15$ from each side of the equation to move constants to the right side of the equation:
$3x = 45$
Divide both sides by $3$ to solve:
$x = 15$
