You can put this solution on YOUR website! Just translate directly into symbols.
Let n be "a number". ----------that's it.
... the only real problem is the ambiguity in your description.
The imprecise language does not distinguish between (6n)+15 and 6(n+15). Where is the boundary of the "and"? Does the description want to mean, 6*n*15? That is, product of 6, n, and 15?
Not really a good exercise problem because it is not precise.