I am having a lot of trouble with this question:

An initially stationary box of sand is to be pulled across a floor by means of a cable in which the tension should not exceed 783 N. The coefficient of static friction between the box and the floor is 0.23. (a) What should be the angle between the cable and the horizontal in order to pull the greatest possible amount of sand, and (b) what is the weight of the sand and box in that situation?

Can someone help me to get started?

Here's how far I have gotten:

Tcos(theta)-mu_{s}(mg-Tsin(theta)=0