The equation "V =3(t-20)^2" models the relationship between the volume of water (V litres) and time (t minutes). According to this model, when will the volume equal 75 litres?

They gave us the answer, "Answer=15; Explanation=Set V=75 and solve for t."

I guess I just can't properly figure out how to isolate t, to get the answer. Could someone show their work and the process they went through to get 15 as the answer?