Converting from symbolic to matrix

[theta]

I have a cell array defined as:
g =

{
  [1,1] =

(2.0)*x1+(40.0)*(-x4+x1)^(3.0)+(20.0)*x2
  [1,2] =

(4.0)*(-(2.0)*x3+x2)^(3.0)+(20.0)*x1+(200.0)*x2
  [1,3] =

-(8.0)*(-(2.0)*x3+x2)^(3.0)-(10.0)*x4+(10.0)*x3
  [1,4] =

(10.0)*x4-(40.0)*(-x4+x1)^(3.0)-(10.0)*x3
}

After substituting particular values if x1,x2,x3,x4 using subs() I got gk
as:
gk =

{
  [1,1] =

306.0
  [1,2] =

-144.0-1.4693486515086033901E-18*I
  [1,3] =

-2.0+2.9386973030172067802E-18*I
  [1,4] =

-310.0
}

Firstly, how do I convert the 2nd and 3rd index values to real values. Tha
imaginary part just comes from nowhere. All my calculations are in real
numbers.

Secondly, how do I convert this cell array gk into a matrix to perform
matrix multiplication. Every time I try to assign one of these elements into
an array I get an error saying wrong type `ex' - What does this mean and
what is its workaround. 

I would be grateful for any help.
Thanks
Apoorv
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