This is a congruent triangle. If we use Pythagorean theorm, we can see that 3^2+y^2=25
y^2= 16
y=4
(y is the base of the small triangle)
and since the big triangle and the small triangle are congruent(the ratios are the same), 3 is half of 6, and so the base of the small triangle is half of the big triangle's.
