The width of a rectangular picture is 13 in less than the length the area of the picture is 30in 2 what is the length of the picture

Let x represent the length of the picture. Then its width is represented by (x-13) and its area will be
  A = length*width
  30 = x(x -13)

This quadratic equation can be put into standard form and factored to find the solutions.
  x² -13x -30 = 0
  (x -15)(x +2) = 0
  x ∈ {15, -2}

A negative length makes no sense, so we conclude ...
  the length of the picture is 15 inches.