Use Moseley's Law for Kalpha. 
nu = (2.47 x 10^15) (Z - 1)^2 
We know that E = h*nu, so... 
nu = E / h 
substitute, rearrange, etc, you get... 
Z = sqrt( (E/h)/(2.47*10^15) ) + 1 
Answer yields 18.98 ... ~ 19, hence Potassium, like I wrote earlier in reference to NIST tables.