a frame holds a picture that is 15 inches long and x inches wide. the frame border is
3 inches wide around the picture. what expression represents the area of the frame border?

So the area of the picture will be 15 * x inches but you have a buffer zone of 3 inches so you must do (15+3) x (x+ 3) and you can multiply this to get 18 x x+ 3 which is 18 x + 54 or 18(x +3)
Hope this helps :))