two charged object are positioned 15cm apart. spere a has a charge 3.6 x 10^-7 C and b has a charge of -8.1 x 10 ^ -7 C. at what point is the field intensity 0 ?

Look for a point along the "line of centers" where the fields due to the two charges are equal and opposite. It will NOT be between the charges, since the charges are of opposite sign.

The zero field point will be closer to the 3.6 x 10^-7 C charge than the other, larger, charge. Imagine than it s distance x past the 3.6x 10^-7 charge, along the line of centers.

Using Coulomb's law

3.6/x^2 = 8.1/(15 + x)^2

(15 + x)/x = sqrt(8.1/3.6) = 1.5
15 + x = 1.5 x
x = 30 cm

The Coulomb constant cancels out.