
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given the universal set $U = \mathbb{R}$, set $A=\{x|x^{2}+3x-18\leqslant 0\}$, $B=\{x|\frac{1}{x+1}≤-1\}$.  
$(1)$ Find $(\complement _{U}B)\cap A$;  
$(2)$ If set $C=\{x\left|\right.2a \lt x \lt a+1\}$, and $B\cup C=B$, find the range of real number $a$.

Present the answer in LaTex format: \boxed{Your answer}
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