If 3 times a number minus 2 equals 13, what is the number?
a. –6
b. 5
c. 2
d. 4

It is very important to read and understand the words of the problem before getting down to solving the given question. Numerous information's of immense importance are already given in the question. Those information's will come in handy while solving the question.
Let us assume the unknown number to be = x
Then
3x - 2 = 13
3x = 13 + 2
3x = 15
Dividing both sides of the equation by 3, we get
(3/3) *  x = 15/3
1 * x = 5
x = 5
So the unknown number is 5. I hope the procedure is not very tough for you to understand.

3x - 2 = 13

Add 2 on both sides

3x - 2 + 2 = 13 + 2
3x = 13 + 2
3x = 15

Divide both sides by 3.

3x/3 = 15/3

x = 15/3

x = 5

Your final answer is A. 5.