verify that the function f and g are inverse of each other by showing f(g(x)) and g(f(x)) =x

f(x)= 6-2/5x g(x)=15-5/2x

just plug and chug:

(I rewrite 2/5x as 2x/5 to avoid ambiguity)

f(g) = 6 - 2g/5 = 6 - 2(15 - 5x/2)/5
= 6 - (30 - 5x)/5 = 6 - (6 - x) = x

g(f) = 15 - 5f/2 = 15 - 5(6 - 2x/5)/2
= 15 - (30 - 2x)/2 = 15 - (15 - x) = x