Whenever there is a number, there is a beauty (old wisdom)

Just check the last 2 digits of the given number . this can be done by checking the last digits of the given 2 factors first one is odd but second is even and the last two digits of number are 28 this clearly says that the odd factor's last digit is 7 and even factor's last digit is 4 so 19 is the answer

The easiest way to tackle this problem is to plug-in the value for $k$ in the equation. Since he gave me the values of $k$ and $k$ satisfies my equation. it means the right value of $k$ should give me both sides equal and in our case $k$ = $19$ works. Also, this is the seventh perfect number, so if you were familiar with these numbers, you would also notice that $k$ = $19$ [another trick that would be helpful]