Slope intercept form: y = mx + b
m = slope
b = y intercept
The first thing you must do is isolate y so that it is by itself on the left side of the equation:
-x = y + 7
Subtract 7 to both sides:
-x - 7 = y + 7 - 7
-x - 7 = y
Flip:
y = -x - 7
-x = y + 7 in slope intercept form is y = -x - 7
This means the slope is -1 and the y-intercept of the equation is (0, -7)