I originally thought that if FC and EC divided by BF and AE are the same then 5.4 divided by 1.2 would mean that the answer is 4.5 but it is not.
Both B and F are 90 degrees, a triangle like this would have to equal 180 degrees so that means that c and e and a are the same degree so the triangle is similar.
Next we have to find the scale factor: BC over FC or 15.9 over 8.7. 15.9 ÷ 8.7=1.83
1.83 X 5.4 = 9.88=X