Dimensional analysis simply means to arrange the units in the problem in a such a way that they cancel each other out and you're left with the units you need in the answer. We do this by using conversion ratios arranged like fractions, then multiplying across left to right.
In your problem, we're given ft/min and need to convert to yd/s. Start with feet and convert to yards for the numerator, then convert minutes to seconds for the denominator: (pretend the parentheses encompass the entire fractions - limitations of the answer box)
39 feet x ( 1 yard ) x (1 min)
1 min 3 feet 60 sec
Notice that feet cancel, and minutes cancel. We're left with yards in the numerator and seconds in the denominator.
Multiplying the numbers, we get 39/180, which simplifies to 0.217.
The final answer is ~0.22 yd/sec.