kill(all);

van=facebook.com/LearningCalculus$

f(a,b,L):=sqrt(2*(a^2+b^2))-L;

assume(L > 0)$

area(a,b,L) := rhs(first(solve(f(a,b,L),b)))^2$
area(a,b,L);

area(3,b,30);