Question 862174
You have a two variable equation {{{z=f(x,y)}}} but you have initial value only for y. 
You need initial values for {{{f(x,y)}}} not {{{x}}} or {{{y}}}. 
Also, what are a and b? 
Is {{{a=f(x0,y0)}}} and {{{b=f(x0+h/2,y0)}}}??
It would be helpful if you could write out the Runge Kutta formula you are to use.
Please repost with additional information.