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