You would need to write at least 11 checks for the second one to be a better option.
Since the first has a lower base cost, we know it is cheaper to start. So, we have to find out when they become even. To do so, we need an equation for both banks. We can model the first one using 5 + .25x (where x is the number of checks) and the second as 6 + .15x. Now we can set them equal to find when they are even.
5 + .25x = 6 + .15x -----> subtract 5 from both sides
.25x = 1 + .15x -----> subtract .15x from both sides
.10x = 1 -----> multiply both sides by 10
x = 10
Since they are even at 10 and better for bank 1 before that, they are all better for bank 2 afterwards.