Another way to attempt this question :

Volume of tank = 34 cm x 52 cm x 16 cm
= 28288 cm³

Since only 56 cubes can be placed into the tank,

28288 cm³ ÷ 56 = 505 cm³ R 8

So this means that one cube cannot be more than 505 cm³ .

Now, we know that the length of the cube is a whole number. Find the length that can give you a volume closest to 505 cm³ .

If length = 6 cm, volume = 6 x 6 x 6
= 216 cm³

If length = 8 cm, volume = 8 x 8 x 8
= 512 cm³ (too high, exceeded 505 cm³)

Try length = 7 cm :
volume = 7 x 7 x 7 = 343 cm³

So the length can only be 7 cm