There are two like terms. 5n and -7n. And one that’s different. 7. Combining just means adding or subtracting. Add the 5n to -7n. And leave the +6. You got it.
We know that V = a^3 and that it's 1000 cubic meters (or unit cubes as long as one unit cube is 1x1x1m). We can write the calculation as attached below. The result (dimensions of one cube house) is 10x10x10 meters.