kill(all)$

van=facebook.com/LearningCalculus$

f(a,b):=a+b=16$
g(a,b):=a-b= 8$

s:solve([f(a,b),g(a,b)],[a,b]);

a:rhs(first(first(s)));

a^2;

solve(f(x,y),y);
solve(g(x,y),y);