Solve for x. show the equation you used and your work.

Here, You need to apply "Proportionality Theorem"
According to this theorem, the ratio of components of a leg would be equal to the ratio of corresponding components of another leg.

In mathematical form, 
TV/VS = TW/WU

Substitute their values, 
14/6 = 21/(x+4)
14(x+4) = 21*6
14x+56 = 126
14x = 126-56
x = 70/14
x = 5

In short, Your Answer would be 5

Hope this helps!