2/3 of 800
The word 'of' generally means multiply, so then
(2/3) * 800
800 is like 800/1, the fraction, and when multiplying fractions we multiply the top of the first and the second together, and multiply the bottom of the first and second together (numerator, denominator).

You don't really need the 800/1, but that might make it more clear exactly what's happening.
Edit: So I'm not sure if it's just me or that LaTeX formatting actually doesn't render. If not,
(2\3)*(800/1) = 1600/3 = 533.333333
Answer: The cause of exponential decay. Sowwy if im wrong
Step-by-step explanation:
An inverse variation function is not the best model because the data points show an exponential decay. The fact that this is true for ALL of the points shown indicates we have an inverse variation of the form x*y = k where k = 60 in this case. ... For any inverse variation, as x increases, y will decrease (and vice versa).
Difference of two squares is of the form (a + b)(a - b) or vice-versa. Here, a = -5x and b = 3.
(-5x - 3)(-5x + 3)
Answer: 3