In order to find the value of x, you first take into account that the angle ∠AED is equal ∠DEC. Furthermore, angle x is equal to ∠AED.
Thus, you have:
∠AED = ∠DEC = x
Next, you take into account that the sum of the angles ∠DEC + ∠BEC is equal to 180°, that is:
∠DEC + ∠BEC = 180
But, angle ∠BEC is equal to 3x - 10
∠BEC = 3x - 10
Then, you sum the algebraic expressions given above, just as follow:
∠DEC + ∠BEC = 180
x + 3x - 10 = 180
You solve the previous equation for x:
x + 3x - 10 = 180
4x = 190
x = 190/4
x = 85/2
Hence, the value of x is 85/2 °