Calculate value of symbolic equation in MATLAB

I have following MATLAB code:

>> syms x
 >> z = 20.*exp(x)+cos(x)
 >> derivative = diff(z)
 

How can I calculate value of derevative for any number, e.g. 6?

Following commands

>> clear all
 >> x = 6
 >> derevative = 20*exp(x)-sin(x)
 

works fine, but if x is symbolic I don't use x = 6 such as above example.

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