The way to deal with such a question is often to break it down into pieces, and write an equation for each piece.
So suppose the whole cake is $c$ grams (c for cake, name the thing you want to find)
Then one seventh of the cake is $s=\frac c 7$ (s for seven)
And one fifth of the cake is $ f=\frac c 5$ (f for five)
And you also have the equation $f=s+80$
That deals with all the information you have been given in an orderly fashion - except it involves a lot of extra variable names, and you only want to know about $c$. You can take any of the three equations and use the others to substitute out $f$ and $s$ to give you a single equation in $c$ alone, which you should be able to solve. In fact the third one is easiest, even though it doesn't involve $c$ at all at the moment. But I'll leave that to you.
I really wanted to point out that it is sometimes easier to write down a number of very simple equations, and solve these, than to try to get directly to a single equation, when it is easy to make a mistake.