10.
In order to find the missing value, we need to divide 173.1, by 17.31:
173.1 / 17.31 = 10
Dividing the product (173.1) by one value in the equation (17.31) can represent what the missing value is, in order for 17.31 to equal 173.1
17.31 · 10 = 173.1
173.1 / 10 = 17.31