How do I calculate the hydronium ion concentration if I know the strontium hydroxide concentration?
Strontium hydroxide is a strong base, so you can calculate $[\ce{OH-}]$ as $\pu{0.02 M}$, then use

$$K_\mathrm{w} = \ce{[OH-][H3O+]}$$ 

$$1\cdot 10^{-14} = 0.02\cdot [\ce{H3O+}] \quad\to\quad [\ce{H3O+}] = \pu{5e-13 M} \quad\to\quad \mathrm{pH} = 12.3$$