They want you to write the number in index form.
The number is 5N^2, where N = 3 x 5^2 x x^3
So:
5N^2 = 5 x (3 x 5^2 x x^3)^2
= 5 x (3^2 x 5^4 x x^6)
= 3^2 x 5^5 x x^6
or a bit neater looking: 3² ₓ 5⁵ ₓ x⁶
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