How to use "fzero" command with a variable(s)?

format long;
lp_upper='besselj(1,x)/besselj(0,x)-besselk(1,sqrt(3.543149609311796^2-x^2))/besselk(0,sqrt(3.543149609311796^2-x^2))*sqrt(3.543149609311796^2-x^2)/x'; 
lpupperfrequency = fzero(lp_upper,2)

I am trying to figure out how to use a variable instead of a constant in the attached code above.

as an example, instead of using 3.543149609311796, I would like to use

lmn = 3.543149609311796;
format long;
lp_upper='besselj(1,x)/besselj(0,x)-besselk(1,sqrt(lmn^2-x^2))/besselk(0,sqrt(^2-x^2))*sqrt(lmn^2-x^2)/x'; 
lpupperfrequency = fzero(lp_upper,2)

when I tried that, it gives me an error.