Question

How can I solve number 4 and 6?

Answer 1

4. Here you would use the Pythagorean Theorem. a² + b² = c²
In this case a and b are the same, so : a² + a² = c² would also work.
a = 5√2 so: (5√2)² + (5√2)² = c²
Do you know where to go from here?