That means the triangle has the side length:
a = 12, b = 12, c = 18 (c is the base)
1. Calculate the area by Heron's formula:
The half perimeter is s = 21
2. The height of the trinagle. Actually there are (normally) three heights in every triangle. Here 2 of the three heights are equal.
From you get
Plug in the values you know to calculate the heights: