(4.a) = 8

I'm going to take a guess here........I think (-2, -4) and (0, 0) are on one line and (4, a) and (b, 18) are on a perpendicular line......so....the slope of the first line is

[-4 -0] / [-2 -0] =  -4/-2 = 2

And for the second line, the slope would be  -1/2  ....so we have......

[18 - a] / [ b - 4]  = -1/2    ...... or.....   

2[18 - a]  = 4 - b

36 -  2a = 4 - b

b = 2a - 32     

Then, we can "choose" any sum we want.....for instance, letting a = 16 means that b  =  0

And their sum is just 16 

[I have no idea if this is the correct interpretation, or not  !!! ]

I have no idea what you re talking about -

Hey Chris, do you understand this one?

Mmm  maybe :))