Okay, here we have this:
Considering the provided figure and information, we are going to calculate the requested measure, so we obtain the following:
So to calculate the length of the base, we will solve in the formula for the area of the triangle, we have:
Triangle Area=(Base*height)/2
Triangle Area*2=Base*height
Base=(Triangle Area*2)/height
Let's replace:
Finally we obtain that the length of the base of triangle is 9 cm.