Start off by defining the variables, now you can do this part in several ways so do the one that suits you.
Let the amount Thomasine has be $ T
Let the amount Louis has be $ L
Let the amount Joshua has be $ J
From the question, we know that Thomasine has twice as much as Louis,
From the question, we know that Joshua has $5 less than Thomasine and Louis together so,
J = (T + L ) - 5 if we replace T by 2L we get,
J = 2L + L - 5 = 3L - 5
So we now have all the amounts with just one letter which is what we want so that we can solve it.