A half life of $73$ hours means that after $t$ hours, what remains is $$ 0.5^{t/73} $$ of the original substance. (Note how, for $t=73$, the exponent becomes $1$, so we are left with $0.5$ of the original substance, exactly like we are supposed to. This is a quick check to make sure we have the right expression.)
Now you want to find the value of $t$ for which $5\%$ of the original substance is left. That means that you want to solve $$ 0.05=0.5^{t/73} $$