RULE is: \[a\,B={{a}^{2}}-1.\,{{b}^{2}}-1\] \[43={{4}^{2}}-{{1.3}^{2}}-1=158;\] \[35={{3}^{2}}-{{1.5}^{2}}-1=824\] \[42={{4}^{2}}-{{1.2}^{2}}-1=153\] \[\therefore \] \[31={{3}^{2}}-{{1.2}^{2}}-1=83\]