One slug would equal 14.59 kilogram and 1ft would equal 0.3048m.
15N would be equal to 15 kilogram meter per second square.
1 slug = 14.59kg
1ft=0.3048m
15 N = 15 kg m / s^2
= 15 * (1/14.59) slug (1/0.3048) ft / s^2
= 3.373 slug ft / s^2
= 3.373 pounds