Question 209958
18) {{{y=-2x-5}}}
for y to be negative here, any x that is positive will do. Even x=0 would make y negative
{{{y = -2*0 - 5}}}
{{{y = -5}}}
You could solve it algebraically this way. In order for y to be negative, it must be "less than 0"
{{{ 0 > -2x-5}}}
{{{5 > -2x}}}
{{{-2.5 < x}}} so any x that is larger than -2.5 will give you a y that is negative. Look at the plot and you can see the same thing
{{{graph(400,400, -10,10,-10,10, -2x-5)}}}

19) y= -4 
This is a horizontal line. So all x, no matter what value has a y = -4


20)
{{{ y=2x-5}}}
In this much like 18. 
{{{ 0 > 2x - 5}}}
{{{5 > 2x}}}
{{{2.5 > x}}}
Any x less than 2.5 yields a y that is negative
{{{graph(400,400, -10,10,-10,10, 2x-5)}}}

21)
{{{y=(3/2)x-1/4}}}
Same idea here
{{{ 0 > (3/2)x-1/4}}}
{{{1/4 > (3/2)x}}}
{{{2/12 > x}}}
{{{1/6 > x}}}
Any x less than 1/6 works
{{{graph(400,400, -10,10,-10,10, (3/2)x-1/4)}}}