To simplify the above expression, let's simplify each radicals at first.
√3 and √6 cannot be further simplify.
Now √48 = √(16* 3)
 =√16 * √3
 = 4√3
Similarly, √54 = √(9*6)
 =√9 *√6
 =3√6
So, now we can write the above expression as:
7√3 - 4√6 + √48 - √54
= 7√3 - 4√6 + 4√3 - 3√6
= 11√3 - 7√6