Answer:
3x + 15
Step-by-step explanation:
Let s be the side of the square.
[tex] \\ \tt{}s {}^{2} = 9x {}^{2} + 90x + 225[/tex]
[tex]\\ \tt{}s {}^{2} = (3x + 15)(3x + 15)[/tex]
[tex]\\ \tt{}s {}^{2} = (3x + 15) {}^{2} [/tex]
[tex]\\ \boxed {\huge \boxed{ \tt{}s = 3x + 15}} \\ \\ \\ \\ [/tex]
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