MATLAB: How can I solve this symbolic expression?

Question

I have spent several hours trying to figure this out but I have not been able to. I have:

syms s K ess;
Gs=(K*(s+12))/((s+14)*(s+18))
Kp=limit(Gs,s,0)
ess=1/(1+Kp)

This results in the symbolic equation:

ess = 1/(K/21 + 1)

I am trying to get the value of K, when ess=0.1

I DO NOT WANT to manually write the expression in quotes like this:

solve('1/(K/21 + 1) = 0.1',K)

I need to do something similar but with the equation I already have in the variable ess. Something like this (which does not work):

solve(ess=0.1,K) %where ess is the symbolic expression I already have

Answer 1

sol=solve(ess-0.1,K)