How do I calculate time dilation for objects moving at different velocities?

[bfr]

Homework Statement

I'm trying to find out how to use the formula t'=t*sqrt(1-v^2 / c^2) for time dilation.

For example, if object A were moving .8c and object B were moving .6c, would I do .8c-.6c=.2c, and substitute that in for v in the time dilation formula to find the time object B observes for object A?

Homework Equations

t'=t*sqrt(1-v^2 / c^2)
v'=(u+v)/(1+(u*v)/c^2)

 

[Dick]

To find the relative velocity you need to use the relativistic law for adding velocities. You don't just add or subtract the velocities. You quoted the formula. Now use it.