Answer :

[tex]x^2+2x-15=0\\\\x^2+2x\cdot1+1^2-1^2=15\\\\(x+1)^2-1=15\\\\(x+1)^2=15+1\\\\(x+1)^2=16\iff x+1=4\ or\ x+1=-4\\\\x=4-1\ or\ x=-4-1\\\\\underline{\underline{x=3\ or\ x=-5}}[/tex]

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