The unit measurement of the rate constant is a guide to tell you the rate order. If it's unit is 1/time, then it is a first-order reaction. From the picture attached, the formula to be used would be:
ln[A] = ln[A₀] - kt
ln[0.21] = ln[A₀] - (1.37)(1.2)
Solving for A₀,
A₀= 1.09 M
Hence, the original concentration of X was 1.09 M.
