To simplify a fraction, you need to find a common factor between the numerator and the denomenator, and divid both parts by that until you cannot anymore.
both 84 and 54 are divisible by 2 ->
84 divided by 2 = 42
54 divided by 2 = 27
now, both 42 and 27 are divisible by 3 ->
42 diveded by 3 = 14
27 divisible by 3 = 9
so we are currently at 14/9. they have no common factor, so we cannot divide them further.
to completely simplify this, we will turn 14/9 into 1 and 5/9 (this is because 9 fits into 14 once, and the remainder is 5).
By finding a common factor of the numerator and denominator. When you find it divide them and keep going until you cant divide them by the same factor any more