Find focal length for meniscus convex

[jwxie]

Homework Statement

Find focal length for meniscus convex (3)

Given r = 15, and n = 1.5

Homework Equations

Lens maker's equation

The Attempt at a Solution

I can find the rest except #3 and #6 (they are opposite sign of each other)

I had (1/r1 - 1/r2) gives zero... which is wrong...
I can solve the rest, so I don't really know what else to show... (so I am sorry for not showing any work...)
I spent two hours literally trying to solve this and no luck

Can anyone please help me? Thanks..

 

[Maddark]

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