To answer, we assume that the surrounding of the frame has the same width, x, such that the dimensions of the outside of the picture frame are
(11 + 2x) and (6 + 2x)
The value of x is calculated via the equation,
(11 + 2x)(6 + 2x) - (11)(6) = 18
The value of x from the equation is 0.5.
Thus, the width of the frame is equal to 0.5 cm.