Question

Well shouldn't be too hard to answer but its just sequences

Answer 1

$\\ \sf\longmapsto f(n)=(-5)f(n-1)+15$

Here

$\\ \sf\longmapsto D_f=(0,\infty)$

$\\ \sf\longmapsto f(1)=3$

$\\ \sf\longmapsto f(2)=(-5)f(1)+15=(-5)(3)+15=-15+15=0$

$\\ \sf\longmapsto f(3)=(-5)(0)+15=0+15=15$

$\\ \sf\longmapsto f(4)=(-5)(15)+15=-75+15=-60$