$$(x+1)+(x+2)=9-(x+3)$$
$x=1$
$$2x+1+2=9-\left(x+3\right)$$
$$2x+3=9-\left(x+3\right)$$
$$2x+3=9-x-3$$
$$2x+3=6-x$$
$$2x+3+x=6$$
$$3x+3=6$$
$$3x=6-3$$
$$3x=3$$
$$x=\frac{3}{3}$$
$$x=1$$
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