PLEASE HELP URGENT I'LL GIVE A LOT OF POINTS Find the explicit formula for the given sequence.

-4, 12, -36, 108,...

a
a_n=5·3^{n-1}

b
a_n=4·3^{n-1}

c
a_n=-4·(-3)^{n-1}

d
a_n=15·3^{n-1}

Afactory buys 10% of its components from suppliers b and the rest from supplier c. it is known that 6% of the components it buys are faulty. of the components brought from suppliers a,9% are faulty and of the components bought from suppliers b, 3% are faulty. find the percentage of components bought from supplier c that are faulty.