First, you have to set 5x equal to (3x+10). They are vertical angles and therefore congruent.
5x = 3x+10 The variable should only be on one side of the equation,
5x - 3x = 10 Therefore subtract 3x from both sides
2x = 10 Divide by 2 on both sides.
x = 5
We're not done yet, since we've found the x-value we have to solve for ∠GEC.
∠GEC = 3x+10 = 3(5)+10 = 15+10 = 25°
∠GEC and ∠FEG form a right angle. This means that the sum of the two angles is 90°
90° = 25° + x In this case, we're using "x" to solve for ∠FEG
90-25 = x We subtract 25 from both sides to get the variable by itself.
65° = x
∠FEG = 65° The Answer is B