What is the value of x?3 4 6 8

The given figure is a rectangle, so opposite sides are equal. 
Set up the equation for the width:
\[10 + 2x = 18\]
\[2x = 8\]
\[x = 4\]
Hence, the correct value of \(x\) is 4.

Supplemental Knowledge

In geometry, a rectangle has opposite sides that are equal in length. This property can be used to set up equations to solve for unknown variables. 
For this problem, the lengths of the sides are given as algebraic expressions. To find the value of \(x\), you need to use the fact that opposite sides of a rectangle are equal.

Everyday Examples

Learning how to construct and solve equations is crucial in many real-life applications, for instance architects and engineers needing to calculate dimensions accurately when creating buildings or machinery designs, needing accurate dimensions calculations that ensure safety and functionality in structures they design. Being adept with algebraic expression manipulation enables this.
Budgeting in everyday life often means setting up equations. Once you know your total budget and the costs associated with specific items, algebra allows you to calculate exactly how much is left over for spending on other expenses.

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