Technically the problem is flawed and the answer can't be calculated; however, I assume you mean that the KCl reacts COMPLETELY and stoichiometrically with the AgNO3.
AgNO3 + KCl ==> AgCl + KNO3
moles AgNO3 = M x L = ??
Using the coefficients in the balanced equation, convert moles AgNO3 to moles KCl (it is 1:1).
Now M KCl = moles KCl/L KCl.