There is no such thing as a formula for deceleration; there is only a formula for acceleration. To find the acceleration, subtract the initial velocity from the final velocity and divide that sum by the time taken. If that formula yields a negative answer, it indicates a negative acceleration, or deceleration.
There are many other formulas related to acceleration in physics. To find the distance traveled, assuming initial velocity, acceleration and time taken are given, the initial velocity can be multiplied by time. That sum can then be added to half of the acceleration multiplied by time squared. There are other more complex formulas that exist, such as the five formulas for constant acceleration. These formulas are designed to describe the motion of a particle that is in a state of constant acceleration.