To subtract vectors, you simply have to subtract correspondent coordinates.
So, if you have

the subtraction is simply

So, in your case, we have

12 more boys than girls
ratio of boys to girls = 7:5
thus 2/12 is difference between boys and girls = 12
total students = 12 ÷ 2/12 = 72
108 centimeters equals 1.08 meters