Put the area before the equals sign, and replace the letter h with the height. For example, if the area is 15 centimeters, and the height is 10 centimeters, the equation would look like this: 15 = 1/2 x b x 10.
Calculate 1/2 x h/1 to get h/2. For example, 1/2 x 10/1 equals 10/2, which equals 5. Now the equation looks like this: 15 = 5b.
To isolate b, divide the entire equation by the number on the same side of the equation as b. For example, 15 = 5b becomes 15/5 = 5b/5. Five divided by 5 equals 1, so b is now alone on one side of the equation, which now reads 15/5 = b.
Divide the numbers on the other side of the equation to solve for b. For example, 15 divided by 5 is 3, so 3 = b. The base of the triangle measures 3 centimeters.