How to i isolate t? (time value)sorry for the messy writing. it says: 6m=(6.5m/s)t+0.5(-9.81m/s^2)t^2

The illuminance of a surface varies inversely with the square of its distance from the light source. if the illuminance of a surface is 120 lumens per square meter when its distance from a certain light source is 6 meters, by how many meters should the distance of the surface from the source be increased to reduce its illuminance to 30 lumens per square meter?