is X=y-1 a function

Answer 1

yes it's a function

for every value of x, there is one unique value of y

f(x) =y = x+1, graphically it's an upward sloping line. It passes the vertical line test. It's impossible to draw a vertical line through the graph and cut it in more than one place. That makes it a function.

it also passes the horizontal line test, making it a "one to one function" for every value of y there is unique value for x