Explanation:
Use the following identities:
sin²x + cos²x = 1
tan x = sin x / cos x
sin²α + cos²α = 1
(3/5)² + cos²α = 1
9/25 + cos²α = 1
cos²α = 16/25
cos α = ±4/5
tan α = sin α / cos α
tan α = (3/5) / (±4/5)
tan α = ±3/4
Therefore, cos α = ±4/5 and tan α = ±3/4, depending on the quadrant that α is in.