How many cabinets must he sell to break even?

  He must sell 7 cabinets.

Step-by-step explanation:

So it gives us the equations y= 400x + 1400  and the equations  y=600x and to find the break even point we need to set the two equations equal each other to solve for x.

400x + 1400 = 600x

-400x               -400x

1400 = 200x

 x = 7    

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.