Triangles VBR and VHA are similar (they both share angle V and have a right angle). So, the ratio of their corresponding sides is the same. Let's take the ratios (Side Opposite Angle V)/(Hypotenuse) for each and set them equal:
18/SQRT(18\^2 + 24\^2 ) = x/(24-x)
Solving for x should be pretty straightforward from there.