secondary 4 chevron_right E Maths chevron_right Singapore

Can anyone explain part (ii) cos for my working (may be wrong) is:

a=dv/dt=2e^(2t)-4e^t

Hence, da/dt=4e^(2t)-4e^t

But how it shows that acceleration of particle is always positive since we do not know the final da/dt value is positive or negative