Question

calculate the pH of a 0.50 m solution of HCN. Ka for HCN is 4.9*10^-10

(The chemical equation for this.)

Answer 1

If you mean m it is a tougher problem. I see you can't seem to capitalize the first letter of a sentence so you PROBABLY M which stands for molarity. (m doesn't, it stands for molality.)
.........HCN ==> H^+ + CN^-
I........0.50....0.......0
C........-x......x.......x
E.......0.50-x...x.......x

Substitute the E line into the Ka expression and solve for H^+, then convert to pH.

Answer 2

Got got that concept.
Found that x=1.56 x10^-5 (x is H+)
PH=-log(H+)
=-log(1.56x10^-5)
PH= 4.80. (Is that correct)