BC ≅ EF (CPCTC)
Set the two side lengths equal to each other.
38.4 = 5x - 10.6
Isolate the x. Add 10.6 to both sides
38.4 (+10.6) = 5x - 10.6 (+10.6)
5x = 49
Isolate the x, divide 5 from both sides
5x/5 = 49/5
x = 49/5
x = 9.8
9.8 should be your answer
hope this helps