Using distance formula, we have
√(x−1)2+(7−15)2=10
⇒(x−1)2+82=102 [Squaring both sides]
⇒(x−1)2=100−64
⇒(x−1)2=62
⇒(x−1)2=±6
if (x−1)=6
then x=7
and, if (x−1)=−6
then x=−5