Brauch hier bitte Hilfe

a=(n1-n2)*(1/r1-1/r2)
a=n1/r1-n2/r1-n1/r2+n2/r2
a=(n1-n2)/r1+n2/r2-n1/r2
a-(n1-n2)/r1=(n2-n1)/r2
1/r2=a/(n2-n1)-(n1-n2)/(r1*(n2-n1))
1/r2=(a*r1-(n1-n2))/(r1*(n2-n1))
r2=(r1*(n2-n1))/(a*r1-(n1-n2))