I know the measure of angle m is 100 degrees. I did this multiple times and J and L are not 80 degrees so what are the measures of angles L and J?

Segment [tex]JK[/tex] and segment [tex]LM[/tex] (the two legs of this trapezoid) are equal in length. Hence, trapezoid [tex]JKLM[/tex] would be an isoscele trapezoid. By symmetry, [tex]m\angle L = m \angle K = 100^{\circ}[/tex].

Line [tex]JK[/tex] traverses line [tex]JM[/tex] and line [tex]KL[/tex]. [tex]\angle J[/tex] and [tex]\angle K[/tex] are a pair of consecutive interior angles as they are both between [tex]JM\![/tex] and [tex]KL\![/tex] and are on the same side of the traversal, [tex]JK\![/tex].

Since line [tex]JM[/tex] is parallel to line [tex]KL[/tex], any pair of consecutive interior angles between these two lines would add up to [tex]180^{\circ}[/tex] (supplementary angles.) Thus, [tex]\angle J[/tex] and [tex]\angle K[/tex] are supplementary angles; [tex]m\angle J + m\angle K = 180^{\circ}[/tex].

Since [tex]m\angle K = 100^{\circ}[/tex], [tex]m\angle J = 180^{\circ} - m\angle K = 180^{\circ} - 100^{\circ} = 80^{\circ}[/tex].