Do you know the Pythagoras' Theorem?
It says that for a right angled triangle, the sum of the squares of two shorter sides of a right angled triangle is equal to the square of the longest side (hypotenuse) of the triangle.
Or, in your case,
Here, you need to find the minimum and maximum length of AC.
Let's start by the minimum.
AB is 5.3 to two significant figures. The mininum value of AB could be 5.25 (when rounded off to 2 sf give 5.3).
BC is 4.8 to 2 sf. The minimum value of BC is 4.75.
From those two values, (2sf)
Using the same logic, the maximum values of AB and BC are 5.34 and 4.84 respcetively, giving a length AC of (2sf)