Half the sum of x and y decreased by 1/3 why

yes because there is exactly one y value for every x value

for function there can't be more than one value for every x value

step-by-step explanation:

to find if any function is an inverse of the other, put the first equation in as x in the next one. then simplify and solve. if at the end you get the statement y = x, then it is an inverse. if you get anything else, it is not.