## Basic College Mathematics (10th Edition)

$\angle$ABC = $\angle$BCD = 42$^{\circ}$ $\angle$ECD = $\angle$ABF = 138$^{\circ}$
$\angle$ABC and $\angle$BCD are congruent angles. Therefore they have the same measure. $\angle$ABC = $\angle$BCD = 42$^{\circ}$ The sum of $\angle$ABC and $\angle$ABF = 180$^{\circ}$ due to $\angle$CBF being a Straight Angle. 180$^{\circ}$ = 42$^{\circ}$ + $\angle$ABF $\angle$ABF = 180$^{\circ}$ - 42$^{\circ}$ $\angle$ABF = 138$^{\circ}$ $\angle$ABF and $\angle$ECD are congruent angles. Therefore they have the same measure. $\angle$ECD = $\angle$ABF = 138$^{\circ}$