Get the equation into slope-intercept form and you will easily see the slope.
(1/8)x + 3y = 3
Solve for y.
(1/8)x + 3y = 3
3y = -(1/8)x + 3 <-- Subtract (1/8)x from each side
y = (-(1/8)x / 3) + (3 / 3) <-- Divide each side by 3
y = -(1/24)x + 1
Now we have the slope-intercept form:
y = -(1/24)x + 1
So, the slope is -1/24.