The answer is √91 feet (which is the same as 9.54 feet).
Imagine this as a right triangle. A length of the foot is actually a hypotenuse (c). The distance from wall's base to the ladder foot is one of the sides of the triangle (let it be a).
So, using the Pythagorean theorem:
c² = a² + b²
It is given:
c = 10 feet
a = 3 feet
b = ?
c² = a² + b²
⇒ 10² = 3² + b²
100 = 9 + b²
b² = 100 - 9 = 91
⇒ b = √b² = √91 ≈ 9.54 feet
