We observe that a line will pass through 3 points, but to find a slope we only need two.
We see that it passes through points (assuming the scale of graph is 1 : 1)
and
.
We can use slope formula that applies for any line and produce the same slope for any two different points on the line
So the slope is -1 also it intersects y-axis at
so the general form of the linear function is
That is in your case
Hope this helps.