Help solve this question (Age of a rock using Radiometric dating)

Calculate the age of a mineral using 235U decaying to 207Pb. Half-life is 713 million years. A laboratory determines that, at present, there are 440,000 atoms of 235U and that the amount of 207Pb indicates that when the mineral crystallized, there were 1,200,000 atoms of 235U. (Assume there was no 207Pb in the mineral at the time the mineral crystallize).
Note: U= Uranium, Pb= Lead.