SOLUTION: The square root of 5x+21=x+3...what is x and how did you figure it out?

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Question 137329: The square root of 5x+21=x+3...what is x and how did you figure it out?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
sqrt%285x%2B21%29=x%2B3
First step, square both sides to get rid of the radical.
5x%2B21=%28x%2B3%29%5E2
Expand the right hand side.
5x%2B21=x%5E2%2B6x%2B9
Bring all terms over to the left hand side.
5x%2B21-x%5E2-6x-9=0
-x%5E2-x%2B12=0
Multiply both sides by -1.
x%5E2%2Bx-12=0
Factor your quadratic equation.
%28x%2B4%29%28x-3%29=0
x=-4 and x=3 are both solutions.
Check your answers.
x=3
sqrt%285x%2B21%29=x%2B3
sqrt%285%283%29%2B21%29=3%2B3
sqrt%2815%2B21%29=6
sqrt%2836%29=6
6=6
True statement. Good answer.
x=-4
sqrt%285%28-4%29%2B21%29=-4%2B3
sqrt%28-20%2B21%29=-1
sqrt%281%29=-1
True statement, since %28-1%29%5E2=1.
Good answer.