(I) kinetic energy of the boy when he hits the ground =potential energy
I. e ½mv²=mgh where g is 10ms-²
m=34kg, g=10ms² , h=3m
=34×10×3
=1020j
(i i) v²=u²+2gh
v²=0²+2×10×3
= 60
v=sqrt of 60
It is really easy if you look up an electron configuration table (one that looks like a periodic table) and then just go down the rows left to right, top to bottom, and just stop when you get to where the element is on the table.
You have to use the equation F=ma and solve for m to get m=F/a.
m=mass in kg
F=force (in this case 350N)
a=acceleration (in this case 10m/s²)
when you plug everything in you should find that m=35kg
I hope this helps.