What is the smallest number to be subtracted from $549162$ in order to make it a perfect square?

We can find the closest perfect square below $549162$ by finding its square root using estimation or long division method.
Let's approximate the square root:

$$700^2=490000$$

$$740^2=547600$$

Let's check $741$:

$$741^2=741\times 741=549081$$

Let's check $742$:

$$742^2=742\times 742=550564$$

Since $549081<549162<550564$, the nearest smaller perfect square is $549081$, which is $741^2$.
The number to be subtracted is:

$$549162-549081=81$$