4) c2 = h2 + b2 -2bx + x2

Expanding the square gives (b − x)² = b² − 2bx + x². Substituting this into h² + (b − x)² produces c² = h² + b² − 2bx + x², so the correct choice is “Expand (b − x)².”

AI-written and checked against the verified answer.