substitute the value for the variable and complete the arithmetic

f(x) = -0.8x(x-9)

so f(0) = -0.8*0(0-9)= 0(-9) =0

f(1) = -0.8*1(1-9) = -0.8(-8) = 6.4

and so on

f(x) = -0.8x(x-9)

so f(0) = -0.8*0(0-9)= 0(-9) =0

f(1) = -0.8*1(1-9) = -0.8(-8) = 6.4

and so on