Please help am confused

Another type of subtraction equation is 16-b=7. explain how you would sole this equation then solve it.

Find the nonpermissible replacement for binthis expression. b2/5b +5

Kristen is wrapping presents for the holidays. she has a box that measures 4 ft. x 2 ft. x 2 ft. how many square feet of wrapping paper does she need to cover the box without overlapping?