Hey there!!
Slope-intercept form :
y = mx + b
m is the slope and b is the y-intercept.
Given equation :
x - 2y = 12
We will have to isolate the y.
... x - 2y = 12
... -2y = x - 12
... y = ( x - 12 ) / -2
... y = x / - 2 + 6
Hence, the slope is -1/2
Hope my answer helps!