(LINEAR MOTION) how do you calculate distance?

It depends on what information you have.  The two formulae you are most likely to use are

x = vt + x_0
x = 0.5*at^2 + (v_0)t + x_0

x_0 is initial position and v_0 is initial velocity.  The first formula assumes constant velocity v and the second formula assumes constant acceleration a.  If you are starting from rest at zero distance, you can drop x_0 and v_0 from the equations.  Note that the first equation is a special case of the second, with a = 0.

d = vt + .5at^2
vf^2 = vi^2 + 2ad (vf = final velocity, vi = initial.)
Either of those two formulas should help out.

You have not stated a valid / complete question.  To calculate ANYTHING, you require OTHER variables to have known (numerical) or generic (algebraic) values.  You would then determine the appropriate formula to use depending upon the relationship between the distance and the other variables you case study would involve.  Without that background info, there is no meaning to calculate anything.  If I make afew assumptions about your case, perhaps you have a car travelling a certain distance one way then it travels another distance, you'd find the total distance by adding the two sections.  If the second distance was in the "backward direction, you'd subtract the two distance sections.  hope that helps.

distance = velocity over time

and

change  in velocity over time