Basic average acceleration confusion

[seagull]

Hi there,

I'm struggling to distinguish why the following two equations are giving distinct answers, when the question explicitly states either could be used:

v=u+at
v^2=u^2+ad

when
v=12 m/s
u=0 m/s
a (average acceleration)= the unknown variable
d=24 m
t=4s

Theoretically, the two should provide the same answer; however, only the former is providing the correct answer (3 m/s^-2), the latter is giving 6 m/s^-2 as the answer. If this is the case, how do I recognise which equation is ideal for the problem in question. If I'm making some clearly idiotic mathematical error, I'm sorry, haha!

Cheers!

 

[NFuller]

v^2=u^2+ad

There is a factor of two missing here, it should be
$$v^{2}=u^{2}+2ad$$