With the information provided, you know that Mina took a total of 9,245 steps in three hours and as you know the quantity of steps she took on the first and second hour, you can subtract the steps she took on the first and second hour from the total amount of steps to determine the steps she took on the third hour:
9,245-2,387-3,872=2,986
According to this, the answer is that Mina took 2,986 steps on the third hour.
