how to understand in a speed distance equation that we have to plus it or minus it 

please give the key words to understand it like usual speed will be plus or minus.

Speed is always positive as it is a scalar quantity. While velocity is vector quantity its magnitude and direction both are taken into consideration. 

So, while using an equation,

 

velocity (v) = displacement ÷ time 

 

Velocity can either be positive or negative. 

 

So, if you consider that the object is moving upward direction is positive then object moving in a downward direction is negative. 

Or if the velocity of an object moving in the right is positive then the velocity of an object moving to the left is negative. 

This same concept of the sign (plus and minus) is applicable to displacement which is again a vector quantity.