how to use plot to get a picture?

Here is my code:

syms W
syms k ut t
for(k=1:1:820)
    W(1,k)=dirac(t-(k-1)*1000)
end
U=(u)  %u is known.
ut=W*U
syms thea1 thea2 x1 x2
thea1=0.00097502323063652798979513487003151
thea2=0.0000069952010986280077295009510419408
[x1,x2]=dsolve('Dx1=-thea1*x1+ut,Dx2=thea1*x1-thea2*x2','x1(0)=0,x2(0)=17.3429')
t=0:1000:820000
plot(t,x2,'-k')

the matlab suggested that:Error using plot Conversion to double from sym is not possible.

But I have gotten the x1 and x2, so how to get the picture of x2 by plot?