If 2/3 of the girls in class had brown eyes and 1/4 of the girls have blue eyes, what fraction of the girls in class have neither blue or brown eyes

Here you want to find a common denominator. 12 is a good one. so you need to find out what the fractions will be if the denominator is 12. 
How to do:
2*4=8
3*4=12
2/3=8/12

1*3=3
4*3=12
1/4=3/12

Now add those to see how many girls have brown or blue eyes.
8/12+3/12=11/12
Now do a whole minus 11/12.
12/12-12/12=1/12
So 1/12 of the girls do not have brown or blue eyes. 

Hope this helps!

The linear equation is: y=ax+b