For 2 rectangular to have the same shape is for them to have the same proportions between sides. Suggest x & a are two different rectangular's lengths and y & b are their widths. For them to have the same shape the following formula must apply.

\(\displaystyle\frac{{x}}{{y}}=\frac{{a}}{{b}}\)

\(\displaystyle\frac{{x}}{{y}}=\frac{{a}}{{b}}\)