Answer: x = 7 (choice C)
"sum of 1 and 3 times a number" means we have the expression 1+3*x for some unknown number x. This is set equal to 22, and we isolate x. The idea is to follow the order of operations PEMDAS in reverse. So start with subtraction and try to undo it. If it's not present, then undo addition, and so on, until x is all by itself on one side.
1+3x = 22
3x+1 = 22
3x+1-1 = 22-1 ... see note 1 below
3x = 21
3x/3 = 21/3 .... see note 2 below
x = 7
note1: Undoing addition. We undo the "+1" by subtracting 1 from both sides
note2: Undoing multiplication. 3x means "3 times x". The inverse of this is division which means we divide both sides by 3
