the formula for area of a rectangle is
Area = length x width
since both the length and the width of the rectangle lie on the same x and y axis, we can find the distance between the width and the distance between the length by subtracting
(-4,9) (-4,-3)
these points lie on the same x axis, so they create a vertical line
9-(-3) = 12
12 units is the distance between them
(-4,-3) (-1,-3)
these points lie on the same y axis, so they create a horizontal line
-1-(-4) = 3
3 units is the distance between them
now that we have the length and the width, we can find the area
A = 12 x 3
A = 36 units²