Hip Flexor Workouts?

The weakest parts of my are my deltoids (but I know many exercises to work that, so I'm good there), and my hip flexors (the right is weaker than the left due to a previous injury, which since then has healed, with some scar tissue which inflames/acts up every now and again, but this isn't why both HFs are weak, they just are).
I only know a few exercises to work the HFs, can someone recommend any equipment free HF exercises?
Much appreciated,
- JECA

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