I know that the area of a rectangle is the product of its length and width. In this case, the length is $x$ and the width is 4, so the area is $x \times 4$. I want to find $x$ such that $x \times 4 = 28$. I can divide both sides by 4 to get $x = 28 \div 4$. I can simplify this to get $x = 7$. 
The area of rectangle $ABCD$ with vertices $A$(0, 0), $B$(0, 4), $C$($x$, 4) and $D$($x$, 0) is 28 square units. If $x > 0$, what is the value of $x$?