Sum of two interiors=exterior
[tex]\\ \rm\bull\rightarrowtail 2x-3+2x+15=7x-18[/tex]
[tex]\\ \rm\bull\rightarrowtail 4x+12=7x-18[/tex]
[tex]\\ \rm\bull\rightarrowtail 3x=30[/tex]
[tex]\\ \rm\bull\rightarrowtail x=10[/tex]
