You want to hang three equally-sized travel posters on a wall so that the posters on the ends are each 3 feet from the end of the wall. you want the spacing between posters to be equal. write and solve an equation to determine how much space you should leave between the posters.

We need to make some assumptions
we don't know a few things:
1. the width of the wall
2. the width of the posters
3. the width of the gap, that we will solve for

alright, so
the width of the wall will be r
the width of each poster will be p
the width of the gap between each is w

so
make an equation for the width of the wall
3 feet on the ends (2 of them) plus the 3 posters plus the 2 gaps between each poster
that means
r=3*2+3*p+2*w
r=6+3p+2w
solving for the width
minus (6+3p) from both sides
r-6-3p=2w
divide both sides by 2


where w is the width of the gap, r is the width from wall to wall, and p is the width of each poster

127 millimeters is the answer