Thank you! understood the mistake) Confused in the task

Sample from task is working:

Letter: m | 12 * 6015 % 26 = 4 == e

Letter: e | 4 * 6015 % 26 = 10 == k

Letter: r | 17 * 6015 % 26 = 23 == x

But trying five letters from first test:

request = "1273409kuqhkoynvvknsdwljantzkpnmfgf" and answer must be "uogbucwnddunktsjfanzlurnyxmx"

Letter: k | 10 * 1273409 % 26 = 18 == s

Letter: u | 20 * 1273409 % 26 = 10 == k

Letter: q | 16 * 1273409 % 26 = 8 == i

Letter: h | 7 * 1273409 % 26 = 23 == x

Letter: k | 10 * 1273409 % 26 = 18 == s

None of the letters match. Maybe I didn't understand the task?