Calculate [OH -] for each of the following solutions, please.?

Question

[H+] = 0.00076 M

[H+] = 5.0 10-7M

a solution in which [H+] is five times greater than [OH -]

Answer 1

I will show you the formula and you are to do the excises.
It is known that [H+]*[OH-] = 10^-14 M^2, so that [OH-] = 10^-14 M^2/[H+].
where 10^a means 10 to the power of a, such that 10^2 =100, and 10^-2 = 1/100.
For the 3rd question, you may use Algebra: let [OH-] be X, and then [H+] = 5X. From the formula I gave you above, 5X*X = 10^-14. You do the math to solve for X.