This is a limiting reagent type problem. All of them can be worked in the following way.
1. Write and balance the equation. You have that.
2a. Convert 28.6 g NH3 to moles. Moles = grams/molar mass.
2b. Convert 25.1 g CH4 to moles.
3a. Using the coefficients in the balanced equation, convert moles NH3 to moles HCN.
3b. Using the coefficients in the balanced equation, convert moles CH4 to moles HCN.
3c. Obviously, one of the answers (either 3a or 3b) must be wrong. The smaller one is ALWAYS the correct value to use and the reagent that produced that value is the limiting reagent.
4. Using the smaller value from 3c, convert moles of HCN to grams. grams = moles x molar mass.
Post your work if you get stuck.