a. \(\frac{3}{4}\times x+\frac{5}{6}\times x-\frac{5}{2}=7\)
\(x\times\left(\frac{3}{4}+\frac{5}{6}\right)=7+\frac{5}{2}\)
\(x\times\left(\frac{9}{12}+\frac{10}{12}\right)=\frac{14}{2}+\frac{5}{2}\)
\(x\times\frac{19}{12}=\frac{19}{2}\)
\(x=\frac{19}{2}\div\frac{19}{12}\)
\(x=\frac{19}{2}\times\frac{12}{19}\)
\(x=6\)
b) \(12\times x-12\times84=0\)
\(12\times x=12\times84\)
\(x=84\)