"when is use the subs synatx to replace the value of a in series1 it is not replacing the vlue of a ."
When you define a=0.56, it overwrites a from symbolic variable to a double variable and thus you can not use it in subs()
There is no need of the line a=0.56. You can directly substitute the value.
In case you want to use a variable to store the value and then substitute, do not name it the same as the symbolic variable, use a different name such as - A or a0 or a_0 etc.
syms x a
series1(x) = a+(2/3)*a*x+(5/3)*a^2*x^2+(1/3)*a^3*x^3
series2(x)=subs(series1,a,0.56)
