If 3x + 4y = 16 and xy = 4; find the value of 9x² + 16y².

The difference between two positive numbers is 5 and the sum of their squares is 73. Find the product of these numbers.

If a = 5, b = -2 and c = -3, a³ + b³ + c³ is equal to :

1. -90

2. 90

3. 60

4. -60