mmmh. Ok the problem states:
prove that the notation is symmetric: for any function f,g,h:N→R>=0
if f(n)=θ(g(n)) then g(n) = θ(f(n))
So, I looked at the definition of Big Theta which is
if d*f(n) <= t(n) <= c*f(n) then t(n)=θ(f(n))
Thus, both conditions need to be true:
t(n) <=...