Step 1) Find angle FDE. This angle adds with the 66 degree angle to get 180 degrees.
(angle FDE) + 66 = 180
(angle FDE) + 66-66 = 180-66
angle FDE = 114 degrees
Step 2) Now use this to find angle E and angle F. They are congruent to each other (E = F) since EF is the base. Use the fact that all three angles of the triangle add to 180 degrees
E+F+(angle FDE) = 180
E+E+114 = 180
2E+114 = 180
2E+114-114 = 180-114
2E = 66
2E/2 = 66/2
E = 33
Since E = F, this means that F = 33 as well.
Final Answer: Angle F is 33 degrees