Let "h" be initial depth (level) of the water in the tank.
And let H be the final depth (level) after immersing (submerging ?) of the cubes.

Surely, we assume (the problem assumes it) that the final level is still lower than 15 cm high.

The initial volume of the water is 25*15*h .
The volume in the tank under the level H is 25*15*H .
Of this final volume 6*5^3  is the volume occupied by 6 cubes.

The "water volume conservation law" says that

25*15*H - 25*15*h = ,  or

H - h =  =  = 2 cm.

Answer.  The level of the water raised in 2 cm after submerging 6 cubes.