Retarding force impacting a stationary object

[JamesTheEng]

Please can you check and if i have gone wrong tell me?

Homework Statement

A mass of 150 kg is dropped onto a pile of mass 1,200kg from a height of 2.5m and drives it 7cm into the ground. calculate (a) the intial velocity of pile and driving weight immediately after impact, (b) the average reisting force exerted by the ground.
150kg
gravity 9.81
1200kg
2.5m
7cm-0.07m

Homework Equations

m1u1+m2u2=m1v1+m2v2
V²=u²+2as
F=ma
Mg-r=MA

The Attempt at a Solution

Here is my attempt not the best.

Intial velocity of pile and driving weight after impact
m1u1+m2u2=m1v1+m2v2
(1200x7)+(150x0)=(1200+150)
8400=1700

(a)v=3.8m/s combired velocity after impact

caculating deccelration

V=0m/s combined mass

V²=u²+2as

0=14.44+(2xax0.07)
0=14.44+0.14a
a=-103m/s²

Resitance

F=MA
Mg-R=MA

(b) (1700x9.8)-R=(1700x-103)
16660-R=-175100
16660+175100=R
R=191700N

 

[gneill]

Intial velocity of pile and driving weight after impact
m1u1+m2u2=m1v1+m2v2
(1200x7)+(150x0)=(1200+150)
8400=1700

(a)v=3.8m/s combired velocity after impact

I can't follow any of that. It doesn't look like any kind of math that I recognize Where does 3.8m/s pop out of?