I need help with this problem please.

A volcanic eruption melts a large area of rock, and all gases are expelled. After cooling, 40Ar (atomic number 18) accumulates from the ongoing decay of 40K (atomic number 19) in the rock (t1/2 = 1.25 109 yr). When a piece of rock is analyzed, it is found to contain 1.00 mmol of 40K and 1.13 mmol of 40Ar. How long ago did the rock cool? Enter your answer in scientific notation.