Question

For a 0.000674 M Mg(OH)2 solution, the hydronium concentration would be -blank- M, the hydronium concentration would be -blank- M, and the pH would be -blank-.

Answer 1

I think you need to proof your your post. I assume you want to know (H3O+), (OH^-) and pH.

0.000674 M Mg(OH)2. So twice that is (OH^-).
(H3O^+)(OH^-) = 1E-14 and you know OH. Solve for H3O^+
pH = -log (H3O^+)
Post your work if you get stuck.