Answer
$x^2+2x$.
Work Step by Step
Divide the given plane figure into two planes as shown in the image file.
Area of the rectangle is side 1 multiplied by side 2.
Area of the figure 1 is $A_1=3\cdot 1=3$.
Area of the figure 2 is $A_2=(x-1)\cdot(x+3)$.
Apply distributive property.
$A_2=x\cdot x+x\cdot 3-1\cdot x-1\cdot 3$
Simplify.
$A_2=x^2+3x-x-3$
Add like terms.
$A_2=x^2+2x-3$
Total area $=A_1+A_2$.
Substitute all values.
$=3+x^2+2x-3$
Simplify.
$=x^2+2x$.
