Answer
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
