book Maths Puzzle Numerical Ability

yes I am trying to add and simplify the following:

(8 + 9i) + (7 - 8i)

multiply
(-2 + 6i)2 power

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book Other Question

3. If a boat goes downstream 72 miles in 3 hours and upstream 60 miles in 6 hours, the rate of the river and the rate of the boat in still water respectively are ________?
a. 7 &1 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.