We have the following:

solving for x

The solution of x is equal to 28, therefore 12 is not a solution of the equation
Answer:
-1952
Step-by-step explanation:
The mean of n numbers is the sum of the numbers divided by n.
You have a mean score of 84 after taking 5 tests. Let x be the sum of scores.

You scored a total of 420 points on five tests.
You need to have a mean score of 85 after taking 6 tests. Let y be the score you need to obtain. Your total score on 6 tests is 420+y.

You need to score 90 points on the sixth test.