(7+√5)√5-(7+√5)7 how to solve this question

Standard deviation of 19,29,21,22,23

Lizette is training for a marathon. at 7: 00 she left her house and ran until 8: 30, then she walked until 11: 30. she covered a total distance of 18 miles. her running speed was six miles per hour faster than her walking speed. find her running and walking speeds in miles per hour.

With this question, the answer with the red arrow is incorrect!