Use this : f = (r1r2)/(n-1)(r2-r1), i had a similar question and in the question they gave 2 separate r values, of 38cm or 59cm, so for the 3rd one you use r1=38cm r2=59cm and then you'll get the correct answer, the 6th one will just be the opposite sign, as r1=59cm and r2=38cm