Orient the triangle so that the base is 6.

You cut the triangle in half, leaving you with a pair of right triangles. The base of each rt triangle is 3, and the hypotenuse is 5.

Now just apply that really old guy's theorem: a^2 + b^2 = c^2

You back into the height by taking (c)5^2 less (b)3^2 (25-9), which is 16. The sqrt of 16 is 4. That's your height.
(4^2)+(3^2)=5^2

Area=1/2bh
= .5 * 6 * 4
= 12sq whatevers

My son's school hit that in 5th here in CA.

Being offended is a natural consequence of leaving the house. - Fran Lebowitz