step 1
In the smaller right triangle of the left we have that
tan(60)=y/a ------> opposite side divided by the adjacent side
Remember that
so
step 2
In the right triangle of the right
we have
tan(30)=y/b
Remember that
so
step 3
Equate equation step 1 and equation step 2
Remember that
a+b=15
substitute the value of b =3a
a+3a=15
4a=15
a=15/4
therefore
b=3(15/4)
b=45/4
b=4 5/4