Answer:
Noah mistakenly added 25 and 7 in denominator instead of subtracting in the third step.
Step-by-step explanation:
Noah had to rationalize the denominator of 
According to him,

Solving the nominator and denominator

Noah did correctly till this point.
In the next step he add 25 and 7 in the denominator, whereas, it should be subtracted.
Hence,
Noah mistakenly added 25 and 7 in denominator instead of subtracting in the third step.