see the attached figure to better understand the problem
we know that
in the triangle ABD
applying the Pythagorean theorem
y²=3³+4²---------> y²=25------------> y=5 units
tan alfa=4/3-----------> equation 1
in the triangle BDC
tan alfa=DC/BD----------> tan alfa=(x-3)/4------------> equation 2
matching equations 1 and 2
4/3=(x-3)/4---------> 4*4=3*(x-3)--------> 16=3x-9---------> 3x=25----> x=25/3 units
the answer is
x=25/3 units
y=5 units
