Finding constant c such that a quantitiy has a t-distribution

#1
Here is my question, and I am not exactly sure where to start:

Question:

Let X1, X2, ... , X5 be a random sample from of size n = 5 from N(0, σ^2). Find constant c so that c(X1 - X2)/√(X^2 (3) + X^2 (4) + X^2 (5)) has a t distribution. How many degrees of freedom are associated with this T?

Note X^2 (3) means X3 squared.

Any direction would be greatly appreciated!

Thanks!