[tex]\displaystyle a).ac+bc=168;~c=7;~a+b=? \\ ac+bc=168 \Rightarrow c(a+b)=168 \Rightarrow 7(a+b)=168 \Rightarrow \\ \Rightarrow a+b= \frac{168}{7} \Rightarrow \boxed{a+b=24} \\ b).ac-bc=540;~a-b=12;~c=? \\ ac-bc=540 \Rightarrow c(a-b)=540 \Rightarrow 12c=540 \Rightarrow c= \frac{540}{12} \Rightarrow\boxed{c=45}\\c).a+b=17;~c=12,~ac+bc=? \\ ac+bc=c(a+b) \Rightarrow ac+bc=12 \cdot17 \Rightarrow \boxed{ac+bc=204}\\ d).a-b=34,~c=16,~ac-bc=?\\ac-bc=c(a-b) \Rightarrow ac-bc=16 \cdot 34 \Rightarrow \boxed{ac-bc=544}[/tex]
