y = ax + b

one way to show the equation of a line is:

y = slope × x + b

the Slope = (y₂- y₁) / (x₂- x₁)

or

(4 - 2) / (3 -1)

or

+1

so put that in and you get

y = 1 × x + b

move x to the other side:

-x + y = b

pick a point and put it into the equation:

-1 + 2 = 1

so

b = 1

-x + y = 1

but people like to make the "x" coefficient positive, so just multiply by -1

┏━━━━━━┓

┃ ✓ x - y = -1 ┃

┗━━━━━━┛

(put both points (1,2) then (3,4) into the above equation to test it.)