A rectangular dog pen has a perimeter of 10 yd and a width of 2yd. What is its area?

Rectangle formulas:

Perimeter = 2l + 2w
Area = lw

Note: l = length of rectangle and w = width of rectangle

Let's look at what you know:
Perimeter = 10 yds
Width = 2 yd

Therefore:
Perimeter = 2l + 2w
10 = 2l + 2(2)
10 = 2l + 4
Subtract 4 from both sides.
6 = 2l
Divide both sides by 2.
3 = l

Now you have length and width.
Area = lw = (3)(2) = 6 sq yds

I hope this helps.