(-6,12,- what is the common difference? is it arithmetic or geometric

This question of yours is not well arranged
In an aptitude test if am given a question like this I wouldn't even solve it because the answer is in the question...
How???

"what is the common difference " then you asked" is it Arithmetic or geometric"

I would have chosen arithmetic because common difference is only typical to arithmetic progression... But this question is a typical Geometric sequence "we use Common ratio"

Common ratio = T₂/T₁
Common difference = T₂-T₁

T₂ = 12 and T₁ =-6

∴Common ratio = 12/-6 = -2
Meaning each number in the sequence in creases by a factor of -2

Multiply T₁ by -2 you'll have 12[T₂]
Multiply T₂ by -2 you'll have -24[T₃] and the sequence continues

Hope this helped?

Mike has a choice of an account that compounds daily and an account that compounds monthly. If they both have the same interest rate, which will earn more interest?

Account that is being compounded on a daily basis will earn more interest. This is because in simple words, one is getting some interest on a daily basis and one will yield an 'interest on interest'. Whereas in a monthly compounded account Mike will earn less interest.