15. triangles find the dimensions of a triangle if the base is 2/3 the measure of the height and the area is 12 square centimeters. (lesson 4-3)

You need to use a variable.
The height is unknown.
Let h = height.
The base is also unknown, but it is known in terms of the height.
The base is 2/3 the measure of the height.
Since the height is h, the base is 2/3 of h, or (2/3)h.
Now we have an expression for the height, h, and an expression for the base, (2/3)h. We can use the formula for the area of a triangle.

A = bh/2

12 = (2/3)h * h / 2

Now you have an equation in only one variable, h, so you can solve for h.
Once you know h, find the base.