Answer:
x = 28 cm
Explanation:
By "similar", they mean the ratio of the two sides are equal; essentially, the large one is a scaled up version of the small one.
1. Write the equation that sets the two sides equal to each other with the ratios shown as fractions. You need to make sure the short side on each of the two shapes are both on the top or both on the bottom of the fraction -- if it's switched, your answer will be wrong
=
2. Then, cross multiply (multiply the bottom number from one fraction with the top number from the other fraction) and solve for x.
3x = 7 x 12
3x = 84
x = 84/3
x = 28 cm
Be sure to include the unit of measurement (in this case, cm) in your answers! Some teachers are picky about that so it's best to be safe