Okay, here we have this:
Considering the provided information and equation, we are going to find an equation that will calculate the completed passes, so we obtain the following:
Then from the given equation we will solve for C and the resulting equation will be the one we are looking for, then we have:

The above equation is the one we are looking for, and it will calculate the completed passes for a quarterback with a given efficiency, number of passing yards, number of passing touchdowns, number of interceptions, and number of attempts.