check the picture below.
notice, the missing interior angle is a linear angle with ∡FEG, therefore, both of them add up to 180°, so the missing interior one is the difference of
[ 180° - (x + 4) ].
also recall that the sum of all interior angles in a triangle add up to 180°.
so, let's add them up then,
[tex]\bf x+2x+~~[180~-~(x+40)]~=~180
\\\\\\
3x+~~[180-x-40]=180
\implies
3x+140-x=180\implies 2x=40
\\\\\\
x=\cfrac{40}{2}\implies \boxed{x=20}
\implies
\measuredangle FEG=(x+40)\implies \measuredangle FEG = 20+40[/tex]
