58 = 50+8
Write 50 and 8 in the boxes in the first row.
7*50 = 350
7*8 = 56
350 and 56 go in the boxes in the second row.
Those results represent the area of each room. To get the grand total area, add them up: 350+56 = 406
This means 58*7 = 406. Use the lattice method or a calculator to confirm. Or use whichever technique you are previously used to.
Check out the diagram below to see all the boxes filled.