This problem is a simple matter of plugging expressions into a formula and doing some basic algebra.
First, ask yourself what's the formula for the area of a rectangle? It's just length times width, or l * w.
They've told you values for each of those variables, it's just in terms of a third variable x. So let's substitute those values in for length and width.
l = 3 + x
w = 2x
So A = 2x(3 + x)
Now, they will want you to put your answer in simplest terms, so we need to distribute the 2x through the parentheses. Remember, an x times an x is x2.
A = 6x + 2x2
One last thing we need to do, though. Generally they'll want you to arrange the terms in your equation in order from highest power to lowest power, so swap those two terms around.
A = 2x2 + 6x
Hope this helps!