Which equation is the equation of the line passing through (0,4) and (8,0)

Ineed with number 17 i don’t get it at alll

If 2(a^2+b^2)=(a+b)^2 then, > a+b=0, > ab=0, > a=b, > 2a=b

Iknow its nothing to do with education, but what type of thing should i wrote here? ? : ) its for mcdonald's btw.