\(a,y=kx\Rightarrow k=\dfrac{49}{-7}=-7\\ \Rightarrow y=-7x\\ b,x=7\Rightarrow y=-49\\ x=\dfrac{1}{3}\Rightarrow y=-\dfrac{7}{3}\\ c,y=-7x\Rightarrow x=-\dfrac{y}{7}\\ y=15\Rightarrow x=-\dfrac{15}{7}\\ y=-\dfrac{1}{84}\Rightarrow x=-\dfrac{1}{84}\left(-\dfrac{1}{7}\right)=\dfrac{1}{588}\)