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a)
$x^2-6x+9=100$
$\left(x-3\right)^2=100$
$\sqrt{\left(x-3\right)^2}=\sqrt{100}$
$x-3=10$
$x=13$
b)
$x^2-6x+5=0$
$x^2-6x=-5$
$x^2-2x\cdot3+3^2=-5+3^2$
$\left(x-3\right)^2=4$
$\sqrt{\left(x-3\right)^2}=\sqrt{4}$
$x-3=2$
$x=5$
c)
$2x^2+12x+8=0$
$2x^2+2\cdot2x\cdot3+8+1=1$
$\left(2x+3\right)^2=1$
$\sqrt{\left(2x+3\right)^2}=\sqrt{1}$
$2x+3=1$
$2x=-2$
$x=-1$