[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

[Bob]

(sorry, correction to my previous post)

Multiplying by 3 gets you:

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

Subtracting them gets you:

15x - 3y = 15
-(7x - 3y = -1)
-----------------
8x = 16

x = 2

---------------------------------

Multiplying by -3 gets you:

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

Adding them gets you:

-15x - 3y = -15
+(-7x + 3y = -1)
-----------------
-8x = -16

x = 2