+501 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?
+600 $\overline{AB}=4-0=4$
$\overline{BC}=x-0=x$
Area = $\overline{AB}\cdot\overline{BC}=4x=28\implies \boxed{x=7}$.
