Don't mate in one!
What move can white make and not mate in one?
Now, to convert your answer to a number:
Let \(C_1\) be the column you are moving from and \(R_1\) be the row you are moving from. And, let \(C_2\) be the column you are moving to and \(R_2\) be the row you are moving to. (Rows 1-8 and columns 1-8) Finally, concatenate your answer as \(C_1\)\(R_1\)\(C_2\)\(R_2\)
For example, if you were to take black's pawn with your rook, you'd put your answer as 7675.