To calculate your grade, you need to first find the percentage of questions you got correct. You missed 6 out of 42 questions, so you got 42 - 6 = 36 questions correct. This means you got 36/42 = 0.857 or 85.7% of the questions correct.
Most grading scales use the following ranges:
* 90-100% = A
* 80-89% = B
* 70-79% = C
* 60-69% = D
* Below 60% = F
According to this scale, a grade of 85.7% would be a B.