Let
x----------> length of the base
y----------> height
we know that
[the area of triangle]=[length of the base]*[height]/2
x=2y
A=225 km²
225=x*y/2---------> 225=(2y)*y/2-----> y²=225--------> y=15 km
x=2y--------> x=2*15------> x=30 km
the answer is
the height is 15 km