How Can I Solve Integration Problems in Mathematica?

[Nusc]

Homework Statement

I'm having problems interating the following function numerically using Mathematica.
efield := 10^15
ion := 10^-15
\[Epsilon] := 8.8541878176^-12
q:=1.60217648710^-19

Homework Equations

x2:= (1/2 - 1/(2*ion) Sqrt[ion^2 - (q^3*efield^2)/(\[Pi]*\[Epsilon])])
x1:= (1/2 + 1/(2*ion) Sqrt[ion^2 - (q^3*efield^2)/(\[Pi]*\[Epsilon])])
Integrate[(3/2)*
Sqrt[1 - q^3*
efield*x^(-1)/(4*\[Pi]*\[Epsilon]) - x], {x,
x1, x2}]

The Attempt at a Solution

I receive the following error message:
Integrate::ilim: "Invalid integration variable or limit(s) in \
{1000000000000000000000000000000\ 1.60218^(19\\:2212)\ \
x,1/2-500<<8>>0000\ <<1>>,1/2+500000000000000 \
Sqrt[1/(100<<25>>000)-<<1>>]}"

I hate computers

 

[EnumaElish]

M. version 6.0.1.0 yields -3.85436*10^36 - 1.25626*10^44 \[ImaginaryI]