we have similar triangles here. x is the base of the small triangle and 6.5 is the height. also, 25 is the base of the large triangle and 20 is the height. since these are similar triangles, their side lengths and angles are proportional, thus we can set up ratios.
can you continue? just plug in the corresponding numbers and solve for x (EDIT: oh, Soroban did that for you already)
EDIT: wow, beaten by Soroban, and he took time to draw the diagram!