Answer :
According to the problem, angles E and F are equal. Angles D and G are equal.
Also, if the trapezoid is isosceles, then FG = DE by definition. So, we express the following
[tex]\begin{gathered} FG=DE \\ 11=a-4 \end{gathered}[/tex]Let's solve for a
[tex]\begin{gathered} 11+4=a \\ a=15 \end{gathered}[/tex]We already know that the sum of all the interior angles is 360°.
[tex]c+c+4c-20+4c-20=360[/tex]Let's solve for c
[tex]\begin{gathered} 10c-40=360 \\ 10c=360+40 \\ c=\frac{400}{10}=40 \end{gathered}[/tex]Then, we find each angle using the value of c
[tex]\begin{gathered} D=c=40 \\ G=c=40 \\ E=4c-20=4\cdot40-20=160-20=140 \\ F=4c-20=4\cdot40-20=160-20=140 \end{gathered}[/tex]