| Using the red gun, first, collect cubes from HMM if you have none. Make more cubes from that. Then start filling out a floor from the space below you where the floor recedes slightly downward. Walk across your expansion bridge. If you're speedrunning, make the bridge go partway, then jump the gap.
This problem is solvable with the yellow gun. Make bridge with 'WTF!?' just same as 'HMM', but it will be more difficult.
It is possible to get across using only the blue gun in a similar way to how 'HMM' can be solved with the blue gun: jump down, collect cubes from 'HMM', build a staircase at the top, and then pillar up and climb on the staircase. With proper preparation, it is possible to collect cubes through the gap intended to be used to make a bridge with the red gun.