let

x = the number of stickers that Davis has, x>0

2x = the number of stickers that Matthew has

each will have 120 stickers means the total number of stickers is 120 + 120 = 240 and

x + 2x = 240

by solving we find:

x = 80

click here to see the step by step solution of the equation:

David has 80 stickers.

Matthew has 2*80 = 160 stickers.

let t = the number of stickers that Matthew must vice to David so that they each have 120 stickers

160 - t = 120

t = 160 - 120

t = 40 stickers

and also 80 + t = 120, indeed 80 + 40 = 120

Matthew must give 40 stickers to David.