[Mr. K]

A

Then you get a +3y and a -3y

Next, you subtract the first equation from the second

The +3y and -3y = 0 and so you are left with an easily solvable equation for x alone

[Bob]

Either A or C will work.

Multiplying by 3 gets you:

15x - 3y = 5
7x - 3y = -1

Subtracting them gets you:

15x - 3y = 5
-(7x - 3y = -1)
-----------------
8x = 6

Multiplying by -3 gets you:

15x - 3y = 5
-7x + 3y = 1

Adding them gets you:

15x - 3y = 5
+(-7x + 3y = 1)
-----------------
8x = 6