4x squared + 15 = x squared + 18x help pls

4x^2+15=x^2+18x

• minus 4x^2 from both sides

15=-3x^2+18x

• minus 15 from both sides

-3x^2+18x-15=0

• take a factor of -3 out of brackets

-3(x^2-6x+5)=0

• sum and product gives factors of 6 and -1
• factorise the brackets

-3(x+6)(x-1)=0

• Null factor theorem means:

x = -6 or 1

\(\begin{array}{rll}4x^2+15&=&x^2+18x\\3x^2-18x+15&=&0\\x^2-6x+5&=&0\\(x-1)(x-5)&=&0\end{array}\\\text{Either one of the factors equal 0.}\\\begin{array}{rl}x-1=0&x-5=0\\x=1&x=5\\\end{array}\)

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