Go step by step to reduce the radical.
√ 128
√16 • √8
8 √2
8 √4 • √2

I think its 8×4√16
Explenation:
√128 = 11.3
√16×√8=6.3
8√2= 11.3
so i think you see the patern there and the answer should be 11.3 or 6.3
8√4×√2= 19
19÷2=9.5
9.5÷2=4.75
since 5 is too big to put in the equation and equal 128 then its 4.
To find the second one just do 4², which is 16
So yeah 8×4√16