I have a lab tomorrow, and I must do some prelab questions for the experiment. It's concerning acids and bases.

In the first question I wrote this reaction, (the first ionization of Phosphoric acid and water) which I think is correct:
H3PO4 + H2O ---> H3O+ + H2PO4-
Next I found the Ka1 constant for the reaction:
Kal= [H3O][H2PO4-] / [H3PO4]

The third question asks: "What would be the pH of the solution when [H3PO4]=[H2PO4-]?"
I do not yet understand the concept of pH very well, and thus I am not clear on what the question is asking; I also have these given values:

Ka1= 7.5 x 10 ^ -3
Ka2= 6.2 x 10 ^ -8
Ka3= 4.2 x 10 ^ -13

I understand what pH is, I just don't know how to use it; pH = -log[H3O], but I am at a loss as to how to utilize this formula.

Thank you!