Question

18 + __ = 0
-18 + __ = 0
-23 + __ = 0
23 + __ = 0
-23
23
-18
18

Explanation:

Step1: Solve for first blank

Let $x$ be the unknown. $18 + x = 0 \implies x = 0 - 18 = -18$

Step2: Solve for second blank

Let $x$ be the unknown. $-18 + x = 0 \implies x = 0 - (-18) = 18$

Step3: Solve for third blank

Let $x$ be the unknown. $-23 + x = 0 \implies x = 0 - (-23) = 23$

Step4: Solve for fourth blank

Let $x$ be the unknown. $23 + x = 0 \implies x = 0 - 23 = -23$

Answer:

$18 + \boldsymbol{-18} = 0$
$-18 + \boldsymbol{18} = 0$
$-23 + \boldsymbol{23} = 0$
$23 + \boldsymbol{-23} = 0$

(Matching pairs: Left top ↔ Right third; Left second ↔ Right fourth; Left third ↔ Right second; Left fourth ↔ Right top)