If you want to get this as line plot, you'll need all values of a and c to be in one array each. If the example you have posted is the real equation, you should simply vectorize it and things become much easier:
b=1:1:4;
a=0:4:16;
[Am,Bm]=meshgrid(a,b);
Cm=Am+24*Bm;
plot(a,Cm)
If this was just sketching the problem and you cannot vectorize your function, you'll need to save all c values:
b=1:1:4;
a=0:4:16;
c=zeros(numel(a),numel(b));
for cntb=1:numel(b)
for cnta=1:numel(a)
c(cnta,cntb)=a(cnta)+24*b(cntb);
end
end
plot(a,c)
