This can be done with the principles of similar triangles. Let x = RH = RB, and find RV in terms of x. This gives you the height and base of triangle BRV both in terms of x. Since you already have the numerical height and base of triangle AHV, you just need to show that the two triangles are similar, which lets you take ratios to get an equation that you can solve for x.
EDIT: I did assume RB is perpendicular to the base, as another commentor pointed out. If this isn't given then you would indeed need more information.