Answer:
DEC+DEF=180
DEC=180-116
DEC=64°
in triangle DCE
angle D+angle C+angle E=180
7y+6+4y+64=180
11y+70=180
11y=180-70
11y=110
y=110/11
y=10°
angle C=4y
=4(10)
=40°



Stationary points occur where the derivative is zero. The denominator is positive for all

, so we only need to worry about the numerator.


for all

, so we can divide through:

But

for all

, so this function has no stationary points...
I suspect there may be a typo in the question.