Question 73414
If we let b=8 then
{{{a=(1/2)b-4}}}
{{{a=(1/2)(8)-4}}}
{{{a=4-4}}}
{{{a=0}}}
If we chose a number less than 8 (say 6) then
{{{a=(1/2)(6)-4}}}
{{{a=3-4}}}
{{{a=-1}}}
So if b is less than 8 then a is negative (choice a)
If you are unsure, you can graph by letting a=y and b=x and you'll see which portions are positive and negative.