Answer
2CO + O2 -> 2(CO2)
Work Step by Step
First we choose to balance C since it appears only once on each side. There is 1 C on the left and 1 C on the right so it is balanced. Next we move to O2. There are 3 (2 + 1) on the left and only 2 on the right. However, we can double the right hand side to give
CO + O2 -> 2(CO2)
so that now there are 4 Oxygens on the right side and 3 on the left. There are also 2 C on the right side so we need to balance that first. This gives
2CO + O2 -> 2(CO2)
now the Cs are balanced and the Os are as well
