☆=a, ★=b, and then the topic becomes

Ab-a=57, since the right side of the equation is a number greater than 50, Ab is at least greater than 57 (because A can't be 0, understand this, the decimal is not 0), and the subtraction is a single digit.

So A can only be a number from 5 to 6, and the maximum value of B is 9, and the maximum value of 9-A is 4, so this subtraction needs to borrow from ten digits, so A should be 6, so this problem becomes

60+B-6 = 57, and b equals 3.

63-6=57