The modulus means you make everything inside it positive because you only take into account the magnitude and not the sign. Since x-9 is modded, that means either the original x-9 is already positive or 9-x is positive. Since RHS=LHS, that means 7x must be either equals to x-9 or 9-x. Then just solve the equation. -3/2 is rejected because if x=-3/2, then 7x is negative but 7x cannot be negative because |x-9| (LHS) must be positive so your 7x (RHS) must be positive