The idea here is that the molar mass of aluminium sulfate,
As you can see,
- two moles of aluminium,
$2 xx "Al"$ - three moles of sulfur,
$color(blue)(3)xx "S"$ - twelve moles of oxygen,
$color(blue)(3) xx 4 xx "O"$
Now, grab a Periodic Table and look for the molar masses of aluminium, sulfur, and oxygen
$M_ ("M Al") = "26.981538 g mol"^(-1)$ $M_ ("M S") = "32.065 g mol"^(-1)$ $M_ ("M O") = "15.9994 g mol"^(-1)$
In order to find the molar mass of aluminium sulfate, you will need to do
$2 xx M_ ("M Al") + color(blue)(3) xx M_ ("M S") + 12 xx M_ ("M O")$
You will thus have
$2 xx "26.981538 g mol"^(-1) + color(blue)(3) xx "32.065 g mol"^(-1) + 12 xx "15.9994 g mol"^(-1)$
which will get you
$M_ ("M Al"_ 2("SO"_ 4)_ 3) = "342.150876 g mol"^(-1)$
Rounded to the nearest gram, the answer will be
$M_ ("M Al"_ 2("SO"_ 4)_ 3) = color(darkgreen)(ul(color(black)("342 g mol"^(-1))))$