The first thing you should know to solve this problem is that the definition of the area of the triangle is:
A =(1/2)* b * h
Where,
b: base
h: height.
Clearing the base we have:
b = 2*A / h
Substituting the values
b = 2*(54) / (9) = 12 m
Answer:
The base of the triangle is 12m long