the given info basically gives us the hypotenuse(7 cm) and one side (3cm which u get by dividing 6 by 2) of a right angled trangle. u need to use the equation a^2+b^2=c^2
7^2 - 3^2 = c^2
49-9 = 40
square root of 40 = 6.3245553203367586639977870888654
this to 2dp is 6.32
hope this helps just tell me if its incorrect
have a great day :)