Explanation

5ˣ⁺¹ − 5ˣ⁻¹ = 5ˣ⁻¹(25 − 1) = 24·5ˣ⁻¹ = 600, so 5ˣ⁻¹ = 25 = 5² and x = 3. Then 10²ˣ = 10⁶ = 1000000. Hence the required value is 10⁶ = 1000000.