First you should find the area of wall he's tiling. You can do it by multiplying its length and height.
[tex]$A_w={L}\cdot{H}=42\cdot29\frac{3}{4}=1249\frac{1}{2}=1249,5$[/tex]
After that, the area of one tile.
[tex]$A_t=L^2=(1\frac{3}{4})^2=3\frac{13}{16}=3,0625$[/tex]
And now remains only to divide area of the wall by area of one tile and that will give us the answer.
[tex]$N=\frac{A_w}{A_t}=\frac{1249,5}{3,0625}=408$[/tex]
