Whenever i try so solve for 's' i get the same error:

syms s

G= 1/((s+2)*(s+1)*(s+3)*(s+7)^2)

G = 

K= -1/G;
dk=diff(K)

dk = 

sol=eval(solve(dk==0,'Real',true))

Error using evalin
Unrecognized function or variable 'z'.

Error in sym/eval (line 13)
s = evalin('caller',vectorize(char(x)));

eval(subs(K,s,sol))

Error using evalin

Unrecognized function or variable 'z'.

Error in sym/eval (line 13)

s = evalin('caller',vectorize(char(x)));

Error in PEC2122 (line 8)

sol=eval(solve(dk==0,'Real',true))

Answer 1

[Walter Roberson]

Never eval() a symbolic expression or symfun or symbolic matrix expression. There is no documented meaning for eval() of any of those. In practice, eval() of any of them is eval() of char() of them . Which is a problem because char() of a symbolic expression does not always result in MATLAB code.

syms s

G = 1/((s+2)*(s+1)*(s+3)*(s+7)^2)

G = 

K = -1/G;
dk = diff(K)

dk = 

sol = vpa(solve(dk==0,'Real',true))

sol = 

vpa(subs(K,s,sol))

ans = 

[Walter Roberson]

By the way, you can get exact solutions for this system:

syms s

G = 1/((s+2)*(s+1)*(s+3)*(s+7)^2)

G = 

K = -1/G;
dk = diff(K)

dk = 

sol = solve(dk==0,'Real',true, 'maxdegree', 3)

sol = 

simplify(sol)

ans = 

Answer 2

[Dyuman Joshi]

Avoid the use eval as much as possible. And it is not required here.

If you want to convert the symbolic numbers to numeric data format, use double.

syms s
G = 1/((s+2)*(s+1)*(s+3)*(s+7)^2);
K = -1/G;
dk = diff(K);
sol = solve(dk==0, 'Real', true)

sol = 

sol = vpa(sol)

sol = 

subs(K,s,sol)

ans =