Suppose a sequence satisfies the given recurrence relation and initial conditions. Find an explicit formula for the sequence.

$a_k=7a_{k-1}-10a_{k-2},$ for every integer $k2$

$a_0=-2,a_1=2$

$b_k=b_{k-1}+6b_{k-2},$ for every integer $k2$

$b_0=0,b_1=3$

$a_k=6a_{k-1}-9a_{k-2},$ for every integer $k2$

$a_0=1,a_1=3$

$b_k=2b_{k-1}-b_{k-2},$ for every integer $k2$

$b_0=1,b_1=4$