Substitute the force-deformation relationships:

${\mathrm{\delta }}_1=\frac{{\mathrm{F}}_1{\mathrm{L}}_1}{{\mathrm{A}}_1{\mathrm{E}}_1} {\mathrm{\delta }}_2=\frac{{\mathrm{F}}_2{\mathrm{L}}_2}{{\mathrm{A}}_2{\mathrm{E}}_2}$

into the geometry of deformation equation:

$\frac{{\mathrm{\delta }}_1}{760 \mathrm{mm}}=-\frac{{\mathrm{\delta }}_2}{1,520 \mathrm{mm}}$

to obtain the compatibility equation:

$\frac{1}{760 \mathrm{mm}}\frac{{\mathrm{F}}_1{\mathrm{L}}_1}{{\mathrm{A}}_1{\mathrm{E}}_1}=-\frac{1}{1,520 \mathrm{mm}}\frac{{\mathrm{F}}_2{\mathrm{L}}_2}{{\mathrm{A}}_2{\mathrm{E}}_2}$