Question 666354
given:

{{{2}}} parallelograms 

one of them has {{{b=3cm}}} and {{{a=5cm}}} 

the other has {{{b[1]=18cm}}}  and {{{x}}} to find

since sides going {{{up}}} and {{{horizontal}}}, we have {{{2}}} rectangles and 

we know that not all rectangles are similar (if the proportions are different, they are not similar)

but in this case, we assume they {{{are}}} similar; so, their sides are proportional

then we have:

{{{b:a=b[1]:x}}}

{{{b/a=b[1]/x}}}

{{{3cm/5cm=18cm/x}}}......cross multiply


{{{3cm*x=18cm*5cm}}}

{{{3cm*x=90cm^2}}}

{{{x=90cm^2/3cm}}}

{{{x=30cm^cross(2)/cross(cm)}}}

{{{x=30cm}}}