In a solution at 25 degrees c, the [h+] is 3.5 x 10^ -6 m. what is the [oh-]?

Hey! I can help you with this question!! 

So you first need to understand that [OH-][H+]=1.0 × 10⁻¹⁴. Remember that 1.0 x 10⁻¹⁴ is equal to Kw just for future reference. So now that you know this you can plug and chug.

(3.5 x 10⁻⁶)×[OH-] = 1.0 × 10⁻¹⁴

[OH-]= (1.0 × 10⁻¹⁴) ÷ (3.5 × 10⁻⁶)
[OH-]=2.85714 × 10⁻⁹

Rounded for sig figs= 2.8 × 10⁻⁹

Had a test on this today! Hope it helps!