the answer is here
let be L the length of each ramp
so the the length of the entire dog walk, including both ramps is D, such that
D=L + L + d, where d=12ft
L=hypotenus, and sin30° = 4.5ft x L so L= sin30° / 4.5= 1/2x 4.5=2.25ft
D=2.25ft+2.25ft +12ft=16.5ft