It depends on what it is meant to be equivalent to!
4x
12
It is equivalent to: 6x^2 -8x -8
x^2 +8x -9 is equivalent to (x+9)(x-1) when factored
5
Oh, what a happy little question! 8x plus 16 is like a beautiful tree with many branches. Some expressions that are equivalent to 8x plus 16 include 2(4x + 8), 4(2x + 4), 16 + 8x, and 8(x + 2). Just like painting a lovely landscape, there are many ways to represent the same mathematical idea.
80*8.8 800*0.88 8000*0.088
8(x+5) is equivalent to 8x+40
It is equivalent to: 32x^2 +88x +21
16
It is equivalent to: 3x+6+5 = 3x+11