One expression could be three x to the power of two. Let me know if that's right :)
The answer is: 6/5
Here is why:
3x - 5y = -3
3(1) - 5y = -3
3 - 5y = -3
3 - 5(6/5) = -3
-3 = -3
So if 6 out of 40 are incorrect, 40-6= 36 must be correct.
In order to gain percentage you do proportions.
36/40 = x/100.
Multiply 36 times 100 then divide by 40, which is 90.
The answer is 90%.
90% of the questions he got were correct.