Limitation of a Formulae

Consider a problem say A car is running 5 m/s on road for 5 seconds. If calculate the acceleration $$ a = \frac{v-u}{t} $$ we get $a = 0 m/s^2$ But if we do this $$ t = \frac{v-u}{a} $$ we get t as infinity or not defined Similary few formulae become useless beyond a certain range. How can we tackle this problem? there are too many formulae and every formula is limited
**Or can we write to find acceleration we first subtract final velcoity by initial velocity and the total is divided by time
Also the formula is not applicable for finding the time when the velocity is uniform
Better still we can write time cannot be found by substracting final velocity by initial velocity and the total divided by acceleration in cases where velocity is uniform** Similar types of problem also methods to solve for time is given in standard school textbook may not be in today's time Such formulae create unsettling doubt in the mind of the pupil which contradicts the belief that (if someone me who watches documentary) fomulae can predict many things. Now one may argue that $$ a = \frac{dv}{dt}$$ then OP may again argue why do we need infinitesimal velocity (or sample as I believe infinitesimal means something very small and used only for testing) in case of acceleration as this formula will also fail to find time in case of uniform acceleration. Thus can I say formula fails to satisfy the definition of acceleration There are many such formulae that are limited.

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