Being z = a + bi, is defined as a conjugate of z the complex 
= a - bi, that is:
z = a + bi 
= a - bi
In practice, to obtain the conjugate of a complex number, we exchange the sign of the coefficient of the imaginary part.
Examples
Geometrically, we can observe that two conjugated complex numbers have, respectively, equal real parts and symmetrical imaginary parts.
Example
z = 5 + 2i and its conjugate 
= 5 -2i
