M3 - Edexcel - SHM

From Edexcel Modular Mathematics Book (Keith Pledger) - Page 62, Qn 9.

Essentially, mass = 0.4 attached to ends of two identical springs, nat length 1.2m, modulus of elastcitiy =12. Other ends attached to points A & B, a distance of 4m apart, and the mass moved so that AC = 1.4m and CB = 2.6.

a) Show SHM
b) Find Max KE

a) Did the first part, but canot check answer (not in back), I have w^2 as 50.

I use Ta = 12(1.6-x)/1.2 for one and Tb = 12(1.6+x)/1.2, take one from the other and make resultant F = ma.

b) I did this part, bu saying max velocity is when displacement is zero, and use v^2=w^2a^2, with a as 0.6 and use KE = 0.5mv^2 -> I get an answer of 6J, in the back it's 3.6J

Where am I going wrong