。
She was disappointed with the result.
Literal
She [topic-は] that result [at-に] disappointment-did.
失望する ('to be disappointed') is the formal/written register cousin of がっかりする. Composed of 失 ('lose') + 望 ('hope'), it literally describes the loss of one's hopes — a heavier, more deliberate word than the casual がっかり with its onomatopoeic 'sigh-deflated' feel. The polite ~ました ending marks this as desu/masu form, fitting for a written report or polite spoken account. The に here marks the trigger of the emotion — Japanese consistently uses に for the thing that prompts a feeling: ~に驚く, ~に困る, ~に怒る all follow the same pattern.