You have the following function f(x):
f(x) = -2/(5x³)
In order to calculate the previous function for x=-8, that is, in order to calculate f(-8) you simply substitute x by -8 into f(x) and do the calculations, just as follow:
f(-8) = -2/(5(-8)³)
First, you solve the exponentiation of (-8)³, the result of this is a negative number, because a negative number exponentiated to a even integer result in another negative number. Then you have (-8)³ = -512
f(-8) = -2/(5(-8)³) = -2 / (5(-512))
next, you multiply 5 by -512:
-2 / (5(-512)) = -2 / (-2560)
next, you can eliminate the minus sign beacuse it is both numerator and denominator, remember that - / - = +
-2 / (-2560) = 2 / 2560
finally, you use a calculator to do the previous operation
2 / 2560 = 0.00078
In order to approximate the result to its nearest tenth you have
0.00078 ≈ 0.0