Answer:
17
Step-by-step explanation:
A straight line equals 180 degrees. The box on the left indicates a right angle which equals 90 degress. The 90 and 39 degress are given, and when added together that equals 129 degrees. If the total line is 180 degress, you would subtract the 129 degrees from it to get 51. This means that 3(x)= 51. You do 51/3 which equals 17, or x=17.
