Look at the two triangles and identify the variable you need to solve for.
Place the variable over one of the other sides of the triangle as if it were a fraction. Take the two corresponding numbers from the similar triangle and do the same thing. Make sure that both fractions match and both are taken from the same place on both of the triangles. Set the two fractions to equal each other as in "3/4 = X/4."
Cross multiply. Multiply the top of one of the fractions with the bottom of the other. Do this for both fractions. As an example using the previous equation you would arrive at "4X = 12."
Simplify your answer. Using the previous example, you will divide by 4. You would arrive at "X = 3" as your solution.