X=sqrt(-3x+40)? Please solve

Square the equation. The absolute value is just since the -3x+40 part could be either negative or positive:
X^2=|-3x+40|

Now you have two options left:
X^2-3x+40=0
OR
X^2+3x-40=0

You can use quadratic formula to solve but you can do this also (break it into polynomials):
(X+5)(X-8)=0
OR
(X-5)(X+8)=0

X=-5 or 8
OR
X= 5 or -8

So, X=-5,5,-8,8

[edit] The other answers forgot that square rooting requires both a positive and negative value.

How embarrassing, the other answers are right, stupid error on my part, hopefully you read this in time.

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square both sides to get rid of the sqrt:
x^2= -3x+40
x^2+3x-40=0
now it's the quadratic equation, so u factor:
(x-5)(x+8)=0
x-5=0 and x+8=0
so,
x= 5 and -8

Square both sides.

x^2 = -3x + 40

Form quadratic

x^2 + 3x - 40 = 0

Factor

(x-5)(x+8) = 0

x=5 or x= -8