Jawabanmu

2014-01-30T10:10:34+07:00
F=m.a
21,5=m.2
m=10,75 kg

F=m.a
F=10,75.4
F=43N
2014-01-30T14:32:48+07:00
Kita buat perbandingan aja,karna massanya sama kan :)
maka 
F1= 21,5 N
a1= 2 m/s^2
F2 = ..?
a2= 4 m/s^2

maka ..
F1/F2 = a1/a2
21,5 N/F2 = 2/4
21,5 N/F2 = 1/2
21,5 N . 2 = F2
43 N = F2 :)