The principal (positive) square root of 54 is 7.35 to two decimal places. There is also a negative square root, which is -7.35 to two decimal places.
One can verify that 7.35 and -7.35 are square roots of 54 by multiplying 7.35 by 7.35 or -7.35 by -7.35, which gives 54 to an accuracy of two decimal places. Most calculators can compute square roots, although calculating a square root without a calculator is a process of trial and error. One could begin by noting that 7 squared is 49 and 8 squared is 64, which implies that the square root of 54 is between 7 and 8.