。
What would my parents say if I were to fail?
Literal
By-chance fail-[conditional-たら] parents [topic-は] how will-say [probably-だろう].
どう言う ('how say' = 'what would [they] say') — Japanese uses how (どう) where English uses 'what' for questions about manner or content of speech. The plain だろう here hedges the question rhetorically — 'I wonder what they'd say.'