Question 499928
The way you have written the equation the exponentiation (squaring) of (x+3) would not affect the constant multiplier 3.  So, my assumption is that the equation is:
{{{3((x+3)^2)=54}}}
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Divide both sides by 3.
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(x+3)^2 = 54/3
(x+3)^2 = 18
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x+3 = sqrt(18)
x = sqrt(18) -3
x = 3*sqrt(2) -3
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Or we could expand the square and use factoring or quadratic equation for the solution. 
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(x+3)^2 = 54
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x^2 +6x +9 = 54
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x^2 +6x -45 = 0
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*[invoke quadratic "x", 1, 6, -45 ]