Solve for x. show the equation you used and your work.

Here, You need to apply "Proportionality Theorem"
According to this theorem, the ratio of components of a leg would be equal to the ratio of corresponding components of another leg.

In mathematical form, 
AB/BD = AC/CD

Substitute their values, 
36/24 = x/18
2/3 = x/18
x = 18*3 / 2
x = 54 / 2
x = 27

In short, Your Answer would be 27

Hope this helps!