The slope of the equation is the gradient of the equation.
y=mx+c
where m represents the gradient (slope)
here, the value that is written with x is 3, but that isn't your gradient. the equation is not arranged in the correct order. you may rearrange it into the same form as y=mx+c.
so, it becomes
3x+y=-5
y=-3x-5
now, compare both the equations. you can deciper that m=-3. that's what you're looking for!