The ISA temperature at FL300 is (30 x -2°) + 15° = -45°
The temperature deviation is therefore (-57.5°) - (-45°) = -12.5°
The temperature correction is 4% for every 10° deviation from ISA which is:
(-12.5°/10) x 30,000ft x 0.04 = -1500ft
The true altitude is therefore 30,000ft - 1,500ft = 28,500ft