Help minimizing a function

Dear Mathematica Experts,
I have a question regarding the Minimize function.
f[x_,y_]:= 3x^2+y+2;
I need to find the Minimum of x only, and I have condition f[x_,y_]=3.84, I used the following script, but it is not working,

f[ x_,y_]:=3x^2+y+2

Minimize[{f[x,y],f[x,y]==3.84},x],

what will be the correct script to do the same?
I am looking forward to hearing from you.

Best.
Melbin

Try:

NMinimize[{ch[a1, x], 10^(-11) < x < 20000}, a1]