We have that the area of a triangle is given by:
[tex]A=\frac{b\cdot h}{2}[/tex]
We replace the values we know:
[tex]12=\frac{6\cdot h}{2}[/tex]
Now, we solve for h:
[tex]\Rightarrow h=4[/tex]
From this, we have that the height of the triangle is 4 units.
