Answer:
The coefficient for Al(s) is 2.
Step-by-step explanation:
__ Al(s) __ O2(g) --> __ Al2O3(s)
__ Al(s) __ O2(g) --> 1 Al2O3(s)
1 Al(s) __ O2(g) --> 1 Al2O3(s)
1 Al(s) + 1.5 O2(g) --> 1 Al2O3(s) [Use a fraction to make it balance]
2 Al(s) + 3 O2(g) --> 2 Al2O3(s) [Convert the 1.5 fraction to a whole number by multiplying all coefficients by 2]