For question 3, you needn't worry about the number of total students asked. The question is really just asking you what's 45% as a fraction, which is 9/20. This is because 45% = 45/100, divided by 5 = 9/20.
For question 6, just substitute in the values. So you do 4(7 × 4 - 3 × -7), which gives you 4(28 - -21). Two minuses make a plus, so it's just asking you to do 4 × 49 (because 28 + 21 = 49). The answer is 196.
