Ineed to evaluate t + (-u) when t = -6 and u = 3

If the line does not have a y int, then it means it does not cross the y axisthat means it is a vertical lineits x int is (3,0)...so this equation for the line is x = 3

if the line does not have an x int, it means it does not cross the x axis...that means it is a horizontal line...its y int is (0,-4)...so the equation for this line is 
y = -4