In a right-angled triangle ABC, the hypotenuse AB is 3√2 cm. The leg is 3 cm. Find the leg of the AC.

Before finding what the second leg, leg AC, of the indicated right-angled triangle ABC is equal to, recall the Pythagorean theorem.
According to the Pythagorean theorem, the square of the hypotenuse is equal to the sum of the squares of two legs.
That is, c² = a² + b², where a and b are the legs of the triangle, and c is its hypotenuse.
By condition, AC and CB are the legs of the triangle ABC.
That is, AC and CB are a and b of triangle ABC.
And AB is its hypotenuse or c.
Now we find out what the leg AC is equal to if AB = 3√2 cm, and CB = 3 cm.
AB² = AC² + CB².
AC² = AB² – CB².
AC² = (3√2) ² – 3².
AC² = 3² × (√2) ² – 9.
AC² = 9 × 2 – 9.
AC² = 18 – 9.
AC² = 9.
AC² = 3².
AC = 3 (cm).
Answer: AC = 3 cm.