what is the rule that represents the function for ​(0,30), (1,25),​ (2,20), (3,15),​ (4,10)

Answer 1

note that y decreases by 5 when x increases by 1
so, start with y = -5x
But, -5*0 = 0, and you want f(0) = 30
so add 30, to get the final rule of
y = 30-5x