We use the thales theorem to solve this question, then it follows that:
[tex]\frac{56}{56+21}=\frac{x}{x+18}[/tex]then:
[tex]\frac{56}{77}=\frac{x}{x+18}\Rightarrow56(x+18)=77x[/tex][tex]56x+1008=77x[/tex][tex]1008=77x-56x[/tex][tex]21x=1008[/tex][tex]x=\frac{1008}{21}=48[/tex]Then the missing value is x=48
