\((28\times15-34\times12)\times a:6+8=24\)
\(\Rightarrow12\times a:6=24-8\)
\(\Rightarrow2\times a=16\)
\(\Rightarrow a=16:2=8\)
\(\left(28\times15-34\times12\right)\times x\div6+8=24\)
\(\left(420-408\right)\times x\div6+8=24\)
\(12\times x\div6+8=24\)
\(12\times x\div6=24-8\)
\(12\times x\div6=16\)
\(12\times x=16\times6\)
\(12\times x=96\)
\(x=96\div12\)
\(x=8\)