Error of sym>convertChar

How can I solve this problem? Thanks a lot.

Answer 1

[Stephan]

syms a b c x
f = str2sym('a*x^2 + b*x + c')

f = 

subs(f,x,5)

ans = 

subs(f,[x a b c],[5 1 2 3])

ans = 

38

[Steven Lord]

Alternately since all the symbolic variables have been defined on the first line:

syms a b c x
f = a*x^2+b*x+c

f = 

subs(f, x, 5)

ans = 

[Kutlu Yigitturk]

Thank you very much.