To determine the slope of a line is best to write the equation on the slope-intercept form.
The general structure of that form is

Where
m is the slope
b is the y-coordinate of the y-intercept
Then to determine the slope of the given equation you have to write it in terms of y

Pass the x-term to the other side

Divide both sides by 10

From this equation, we can determine that the slope corresponds to the coefficient that multiplies the x-term

The slope is m=-2