Work out change the subject of a formula instantly with clear inputs, formula shown and shareable results.
Changing the subject means rewriting a formula so a different letter stands alone. Using v = u + at as the model, each rearrangement isolates one variable by undoing the operations applied to it.
The four forms
v = u + at, u = v − at, a = (v − u)/t, t = (v − u)/a
v = 5 + 21 = 26, and rearranging for a gives (26 − 5)/7 = 3 as expected.
With no acceleration v equals u for every t, so t is not determined — dividing by zero reflects that.