Greetings!
By using the equation for the area of a triangle you can find the base size. The equation is:
[tex]\frac{b * h}{2}[/tex]
So to rearrange to make b the subject:
[tex]b = \frac{2a}{h}[/tex]
Where a is the area and h is the height. So plug the values into the equation:
[tex]\frac{2 * 18}{6}[/tex] = 6
So the base length is 6cm!
Hope this helps!