# Chapter 1 - Section 1.2 - Informal Geometry and Measurement - Exercises: 37

x = 9

#### Work Step by Step

Step 1: Begin with the equation AB + BC = AC Step 2: Substitute x for AB, (x+3) for BC, and 21 for AC. So, x + (x + 3) = 21 Step 3: Simplify the left side of the equation. 2x + 3 = 21 Step 4. Subtract 3 from both sides. 2x + 3 - 3 = 21 - 3 Step 5: Simplify. 2x = 18 Step 6: Divide both sides by 2. $\frac{2x}{2}$ = $\frac{18}{2}$ Step 6: Solve. x = 9

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