It is a known fact that there are 6 fewer boys than girls in Sophies class. We need to write an expression based on thi fact. It is also given in the question that there are "g" girls present in the class of Sophie. so there are several factors that we already know from the question given.Now
Number of girls in sophies class = g
Let the number of boys in Sophies class - b
Then the expression for the number of boys will be
b = g - 6
This is the final and only expression that can determine the number of boys in Sophies class.