ANSWER
C. 23
EXPLANATION
We are given that Z is the midpoint of AC and Y is the midpoint of BC.
According to the midpoint theorem of triangles, the midsegment of a triangle (YZ) is equal to half the length of side parallel to it (AB)
This means that:
[tex]YZ\text{ = }\frac{1}{2}AB[/tex]
So, we have that:
[tex]\begin{gathered} 21\text{ = }\frac{1}{2}(2x\text{ - 4)} \\ 21\text{ = x - 2} \\ \text{Collect like terms:} \\ \Rightarrow\text{ x = 21 + 2} \\ \text{x = 23} \end{gathered}[/tex]
The answer is Option C.
