Answer: 15 is the correct answer.
Explanation: What I did is the standard way of handling direct variations.
You could set up a ratio .....
y1/y2 = x1/x2
in your case x1=3 and y1 =21
find x2 when x2 = 105
21/105 = 3/x2
cross-multiply
21x2 = 315
x2 = 315/21 = 15