Ok so what even is this? I'd like a full explanation so that I can answer it myself it in the future. Thanks!

The area of the picture is , (20-2x)(12-2x)=84 i.e, Option A

Step-by-step explanation:

Given,

The length of the outer rectangle = 20 cm

The width of the outer rectangle = 12 cm

The area enclosed by the frame = 84 sq cm

Formula

The area of a rectangle = Length × Width sq m

As per the figure,

The length of the picture = 20-2x [ the frame is x cm wide so, we subtract 2x for both side]

The width of the picture = 12-2x

So, the area of the picture is = (20-2x)(12-2x)

Hence,

(20-2x)(12-2x)=84