Question

Solve 3x^2 + 18x + 15 = 0 by completing the square.

Answer 1

You made a mistake in your second line.

You put $3x^{2} +18x$ in brackets, dividing the $3 x^{2}$ by 3, but you didn't divide the 18x by 3.  

So you ended up with 3 times 18x in the end.    

You should divide the 18x by 3 too, so you end up with $3( x^{2} +6x)+15=0$