Answer:
Hint: You are essentially using the transitive property of equality; "If a = b and b = c, then a = c". (Also look at the step by step explanation for more elaboration).
Explanation:
Since we know that line AC is equal to 120, that allows us to add lines AB and BC to give us 120. Simply put, 4x + 6 + 7x + 15 = 120. I got 4x + 6 from line AB and 7x + 15 from line BC. You are essentially using the transitive property of equality; "If a = b and b = c, then a = c". All that is left to do is solve to find x and substitute it into 4x + 6. (Sorry if I may have worded it wrong, like putting lines instead of segments. I have been forgetful of what term to use). Hoped it helped. :)
P.S, If you still need help, please respond back.