There are different types of equations of lines.
There are these three main forms, but I'll focus on just 2:
Slope-Intercept Form:
y = mx + b
Where,
"m" = slope
"b" = y-intercept
Point-Slope Form:
y - y1 = m(x - x1)
Where,
"m" = slope
"y1" = A y point on the graph
"x1" = A x point on the graph that correlates with the y value.
y = 6x - 12
Is in the same form of Slope-Intercept:
So,
y = 6x - 12
m = 6
b = -12
So our slope is,
6
Keep in mind if it was a fraction (Example: 6/5, then the slope is 6/5) the slope will be the entire fraction.
Also remember slope is the RISE over the RUN meaning it is the y value over the x value. So in this case we would go UP 6 and right 1 (because 6 is understood as 6/1).