Hello!
Getting infinitely many solutions from an equation would be getting a solution that is true but does not include the value of a variable. An example of getting an equation with infinite solutions would be 1=1, or 15.6=15.6.
Let's try to get the same thing on both sides of the equation.
8x-16=8x+k
If we want to get the other side to equal 8x-16, we will need to have k=-16.
8x-16=8x-16
Therefore, our value of k is -16.
I hope this helps!