How is the answer with a power of 15? i got -2 the first time

It's not entirely clear what the full problem is as it is cut off; however, it looks like you're using the speed of light (c) which is 3.00*10^8 meters per second. So you would use that instead of 3.00*10^(-9). 

Doing so yields the following
c/lambda = (3.00*10^8)/(1.26*10^(-7)) = (3.00/1.26)*10^(8-(-7)) = 2.38*10^15
which fits with the result you have on the right side

There are 48 blue cards total