How do you solve this problem??

50 mL 0.100 M NaOH is mixed with 50 mL 0.050 M HCl. What is pH is the resulting solution?

What do YOU think ?
While we're happy to HELP we wont do all the work for you.
Show us what you have done and where you are having problems..

Look at the concentrations. What will happen if mixed? What will be left over after neutralization?

Well, to calculate the pH you take pKa + log([A−]/[HA]), and so you have to calculate the pKa, which is -logKa and that's where the problem is. I don't know how to calculate the Ka.

You have a computer, so find the generic formula for calculating ka