This equation is written in standard form, so we need to change it into slope-intercept form.
3x = -y - 5
3x + y = -y + y - 5
3x + y = -5
3x - 3x + y = -5 - 3x
y = -5 - 3x
y = -3x - 5
The slope is classified as "m" in this type of equation, so, the slope is -3.
Best of Luck!