a question about differentiation

P is a point 22m due east of a fixed point O and Q is a point 14m due south of O. A particle A starts at P and moves towards O at a speed of 4m/s while a particle B starts at Q at the same time as A and moves towards O at a speed of 3m/s. **Find an expression for the distance between A and B ***t* seconds after the start. Hence, find the value of t when the distance between A and B is a minimum and find this minimum distance.

i can't figure out how to relate them and make an expression.

i think i can do the latter part once the expression is found.