Answer
$(x^2-7x) \text{ square units}$
Work Step by Step
The shaded region is a trapezoid with $h=(x-7), b_1=x,$ and $b_2=3x.$ Using $A=\dfrac{h(b_1+b_2)}{2}$ or the formula for the area of a trapezoid, then
\begin{array}{l}\require{cancel}
A=\dfrac{(x-7)(x+3x)}{2}
\\
A=\dfrac{(x-7)(4x)}{2}
\\
A=\dfrac{(x-7)(\cancel2^2x)}{\cancel2}
\\
A=x(x-7)
.\end{array}
Using the Distributive Property which is given by $a(b+c)=ab+ac,$ the expression above is equivalent to
\begin{array}{l}\require{cancel}
A=x(x)+x(-7)
\\
A=x^2-7x
.\end{array}
Hence, the area is $
(x^2-7x) \text{ square units}
.$
