Answer:
x = 10
Step-by-step explanation:
Let's draw the center of the circle S and the segments SE and SD.
SE is perpendicular to EF (as EF is tangent to circle).
That means FED + DES = 90deg
We also know that DSE = 120deg
And because the SED is an isosceles triangle, we know that angle measures for DES and SDE are equal.
DES = (180deg - 120deg) / 2 = 30deg
FED = 90deg - DES = 90deg - 30deg = 60deg
7x - 10 = 60
7x = 70
x = 10
