3x + y = -4
what's the slope and y-intercept of this equation?

The slope is -3 and the Y intercept is -4
This is because since the equation is in standard form you have to change it to slope intercept form. Like so;
3X + Y = -4
-3x          -3x
   
            Y- intercept
Y = -3X - 4 
     Slope

The slope would be 3x and your y-intercept would be -4. Why? Because remember the equation is y=mx + b(m-moves by, b-where your slope begins)

What can we do about it

Iwould say false, not positive but if prices went up for both stores, technically there will be less buyers?