To solve 9x^2 + 16 = 0:

9x^2 + 16 = 0
9x^2 = -16
x^2 = -16/9
x = ±√(-16/9)
x = ±(4/3)i

Therefore, the solutions are x = (4/3)i and x = -(4/3)i.

To solve -3x^2 + 4x = 0:

-3x^2 + 4x = 0
x(-3x + 4) = 0
x = 0 or -3x + 4 = 0
-3x = -4
x = 4/3

Therefore, the solutions are x = 0 and x = 4/3.

To solve 2x^2 - 7x - 4 = 0:

2x^2 - 7x - 4 = 0
(2x + 1)(x - 4) = 0
2x + 1 = 0 or x - 4 = 0
2x = -1 or x = 4
x = -1/2 or x = 4

Therefore, the solutions are x = -1/2 and x = 4.