What equation can i write to represent the function?

From an x-value to the next x-value, it goes up 1.
From a y-value to the next y-value, it goes up 3.
Since each equal change in x causes an equal change in y, it is a linear function.
We need the slope and the y-intercept, then we can use y = mx + b to write the equation.
What is the ordered pair just before (2, 5)?
Go down 1 in x and go down 3 in y to get (1, 2).
What is the ordered pair just before (1, 2)?
Go down 1 in x and go down 3 in y to get (0, -1).

From point (0, -1), you get that the y-intercept is point (0, -1), so b = -1.

Now we use two points to get the slope.
A difference of 3 in y gives a difference of 1 in x.
Slope = m = (difference in y)/(difference in x) = 3/1 = 3

y = mx + b

y = 3x - 1