book Maths Puzzle Numerical Ability

How to solve:

(3x-2)(x+5)

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Hello...

I am having some trouble with a proof. I am to use:

(a + bi)(a - bi)

The problem asks:

Show that for a + bi not equal to 0 this product is a positive real number.

Thanks for your help.
the conjugate of
sqare_root(2)-square_root(3) is:
square_root(2)+square_root(3).

But can it be:
-sqare_root(2)-square_root(3)?