μ = –10 cm; v =?, f = 15 cm

Using lens formula

1/f = 1/v + 1/u

1/v = 1/f - 1/u

1/v = 1/15 - (1/(-10))

1/v = 1/15 + 1/10

1/v = 5/30

1/v = 1/6

∴ v = 6 cm