The ratio is supposed to be successful field goals to attempts which would be 813 successful field goals : 1,422 attempts.
This ratio equals the fraction 813/1422 and 813 is a prime number so that is the simplest form of the fraction.
The decimal form of a fraction is the quotient between the numerator and denominator. 813 ÷ 1422 = 0.571729957806. The question says to round to the nearest thousandth, so 0.572.