Draw a straight line from the mid point of one side to the opposite vertex.
Use the Pythagorean Theorem to find the length of this line. Let b = this line (the height).
a^2 + b^2 = c^2
3^2 + b^2 = 6^2
9 + b^2 = 36
b^2 = 27
b = 5.2
Now use this formula to find the area of this triangle.
A = 1/2bh