Please help.

Dear Student ,
Here in this case from the given data ,

A c c e l e r a t i o n = f = \frac{v^{2} - u^{2}}{2 S} = \frac{0 - 400}{2 \times 40} = - 5 m / s^{2} (n e g a t i v e s i g n d e n o t e s t h e d e c e l e r a t i o n) N o w, i f c o e f f i c i e n t o f f r i c t i o n b e μ b e t w e e n t h e b l o c k a n d s u r f a c e t h e n, μ = \frac{F_{f r i c t i o n}}{R} = \frac{m f}{m g} = \frac{f}{g} = \frac{5}{9 \cdot 8} = 0 \cdot 51

Regards