Can someone please help me with a problem?

I'm studying op-amps and can't quite figure this one out. It's analyzing a noninverting op-amp circuit to find the gain. You shouldn't need any electronic knowledge to figure this out - it's just algebra.

Here goes:

V_{in}= V_{out}R_{1}/(R_{1}+ R_{2})

gain = V_{out}/V_{in}= 1 + R_{2}/R_{1}

I cannot figure out, no matter what I try, how to get from the first equation to the second. Can someone please give me a step-by-step solution?

Attached is a copy of the entire text for context.

Thanks!