Total number of questions are 90, and 5 marks are given for every correct answer.

So, maximum marks possible = $90 \times 5 = 450$

Now, 2 marks are deducted for every wrong answer. Essentially, it means that every incorrect answer will decrease the maximum score by $5 + 2 = 7$ marks.

We know that, the student got 387 marks.

So, reduction from maximum marks = $450 - 387 = 63$

Hence, the number of incorrect responses = $63/7 = 9$