The original equation is:
5 x^2 + (10 x^2 + 10) = 3 x^2 - 5
The first step the person did was:
15 x^2 + 10 = 3 x^2 - 5
so in this first step the person used the associative property of addition by associating 5 x^2 with 10 x^2 and combining them to give 15 x^2.
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