Answer
$2r^{2}-5r-12$.
Work Step by Step
$A=a\cdot b$
...the area of a rectangle.
...substitute $(2r+3)$ for $a$ and $(r-4)$ for $b$ in the formula.
$A=(2r+3)(r-4)$
...use the FOIL method.
$A=(2r)(r)+(2r)(-4)+(3)(r)+(3)(-4)$
...multiply coefficients and add exponents.
$A=2r^{2}-8r+3r-12$
...add like terms.
$A=2r^{2}-5r-12$
The area of the stamp is $2r^{2}-5r-12$.
