Answer:
11
Step-by-step explanation:
Let e represent the number of students with blue eyes. Then the number of students with blond hair is 2e. The total number of students is ...
3 + e + 2e -6 = 30
We subtracted 6 because the expressions e and 2e cause the 6 students with both blond hair and blue eyes to be counted twice.
3e = 33 . . . . add 3 and simplify
e = 11
11 students have blue eyes.