Look up Heron's formula for finding the area of a triangle given the length of each side. Using this formula requires you to first find the "semi-perimeter" of the triangle. In Heron's formula the semi-perimeter is usually represented by the letter "s" and the sides by a,b,c. The "a" in the formula is different than the "a" in your problem.
For your problem the semi-perimeter will be 3a/2. Substitute what you are given into the formula and solve for your "a".