The answer is 2.
48/x(9+3)=288
48/12x=288
48=288*12x
48/288=12x
1/6=12x
0.16667=12x
0.16667/12=x
x=0.0138888
Putting the x back in:
48/0.0138888(9+3)=x
48/0.0138888(12)=x
48/0.1666667=x
288=x
48/x(9+3)=2
48/12x=2
12x=48/2
12x=24
x=24/12
x=2
You beg the question in your first step. You assume that (9 + 3) is in the denominator of the expression. If you express this expression in a linear fashion (entering into a calculator or programming a computer), you need to add parentheses to make this so.
48/(x(9 + 3))