One end of a horizontal string of linear density 6.

One end of a horizontal string of linear density 6.9×10−4 is attached to a small-amplitude mechanical 150- oscillator. The string passes over a pulley, a distance away, and weights are hung from this end. What mass must be hung from this end of the string to produce one loop of a standing wave? Assume the string at the oscillator is a node, which is nearly true.