after we make the equation equal to zero, by -15 from both sides it not clear what rules i need to use to seperate into two factors.

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Basically its took up this format:ax^2 + bx + c = 0Youre looking to find two numbers that multiply to make "c" and add up to make "b".so your's was x^2 + 2x -15You need to find two numbers that add up to 2, and multiply to make -15.Since the multiply one's negative, one of the numbers has to be negative, and after some figuring out, youll realise it's 5 and -3, like she said in the video. -3 x 5 = -15-3 + 5 = 2You'll get used to it once you do it a few times.The reason why it goes in brackets is to seperate them to get the theory that something x 0 = 0(x + 5)(x - 3) = 0So (x + 5) = 0 and/or (x - 3)  = 0Then you just solve them to isolate x:x + 5 = 0  ->   x = -5x - 3 = 0   ->  x = 3Most of this is repeating the viddy I know but if this didn't help i'll faint.