Two triangles than make the whole triangle are proportional then
[tex]15\longrightarrow30[/tex]
and the upper side of the left triangle is x
we can calculate the lenght of upper triangle
[tex]36-15=21[/tex]
then
[tex]21\longrightarrow x[/tex]
we join the relations
[tex]\begin{gathered} 15\longrightarrow30 \\ 21\longrightarrow x \end{gathered}[/tex]
we use cross-multiplication. The product of 21 by 30 dividing between 15 is equal to x
[tex]\begin{gathered} x=\frac{21\times30}{15} \\ \\ x=42 \end{gathered}[/tex]
the value of x is 42 units
