Does any one understand this!

let ([0,1], F, u) be a probability space, where F= Bn[0,1],Borel field intersection [0,1], and u is lebesgue measure. Show that mapping

S: [0,1] --> B (Borel)

x---> [2x+1, 4-x] is a pre-random set. Then, find shading of S.