You need to solve for D? You don't show a D in your expression. Furthermore, you don't solve expressions. You need to have an equation to solve for an unknown. I think what you need to do, is copy the problem word for word from its source, leaving nothing out. Then we can help you get organized, which is a prerequisite to being successful in math. With regard to "I do not under stand (sic) how you multiply the top of the fraction by the bottom of the fraction" it goes like this.
We know that anything times 1 is that thing itself. We also know that any non-zero number divided by itself is 1. That is, a fraction with the same numerator and denominator is equal to 1. Therefore, given a fraction such as m/n we can do the following...
\(\displaystyle \frac{m}{n}= \frac{m}{n}\cdot 1 = \frac{m}{n}\cdot \frac{n}{n}= \frac{mn}{n^2}\)
Furthermore, you need to start using the language of mathematics if you want to understand what is going on. The "top of a fraction" is called the numerator. The "bottom of a fraction" is the denominator.
If you need more help, please show us your work. We can then, hopefully, identify where you need help.