The base can be any side of the triangle. However, you do need to pick one side because you measure the height from that side. If the height is given, chose the side that the height divides.
If the length is not provided, you need to measure the length. You can use whatever type of length measurement you want, as long as you are consistent throughout. For an example, the base of one triangle is 3 inches.
The height is the length from the base to the angle opposite. Draw the height in to make it easier to measure.
If the measurement is not provided, measure the height of the triangle. For an example, say the height is 4 inches.
Next, multiply the two numbers together. Multiplying 3 inches by 4 inches results in 12 square inches.
Divide the answer from the last step by 2. In the example, the result is 6 square inches, which is the area of the triangle.