financial mathematics problem

Problem 4. Define by Pr the price of the European put option in the GBM model as the function of x=the value of the underlying stock. In other words, Pa = ENI- In(x/ K) + (r+ 02/2)T In(x/ K) + (r - 02/2)T OVT - Ke TN- - x + Ke-T. OVT Show that Pr is a decreasing function in r