Answer
$i = -26~cm$
Work Step by Step
We can find $i$:
$\frac{n_1}{p}+\frac{n_2}{i} = \frac{n_2-n_1}{r}$
$\frac{n_2}{i} = \frac{n_2-n_1}{r} - \frac{n_1}{p}$
$\frac{1.0}{i} = \frac{1.0-1.5}{30~cm} - \frac{1.5}{70~cm}$
$\frac{1.0}{i} = -\frac{0.5}{30~cm} - \frac{1.5}{70~cm}$
$\frac{1.0}{i} = -\frac{3.5}{210~cm} - \frac{4.5}{210~cm}$
$\frac{1.0}{i} = -\frac{8.0}{210~cm}$
$\frac{i}{1.0} = -\frac{210~cm}{8.0}$
$i = -26~cm$