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Replace the == by = in the lines

e_0 == 8.854 * 10^-12
e_s == e_0 * 3.5
e_m == e_0 * 81

and your code will work. Indeed == is the comparison operator, not the assignment one, which is ⁼.

Besides, you may redefine p as a callable symbolic variable:

p(e, Q, r) = 1/(4*pi*e)*Q/r

so that the definitions of p_1,...,p_4 become simpler:

p_1 = p(e_m, 1, x)
p_2 = p(e_m, -1, d_e-x)
p_3 = p(e_Ersatz, -1, sqrt((2*d_k)^2+x^2))
p_4 = p(e_Ersatz, 1, sqrt((2*d_k)^2+(d_e -x)^2))