\(a,x\times15-12,3\times x=81\)
\(x\times\left(15-12,3\right)=81\)
\(x\times2,7=81\)
\(x=80:2,7\)
\(x=30.\)
\(b,x:2,5-4,4=3,8\)
\(x:2,5=3,8+4,4\)
\(x:2,5=8,2\)
\(x=8,2\times2,5\)
\(x=20,5.\)
\(c,37-x+21,5=35,8\)
\(37-x=35,8-21,5\)
\(37-x=14,3\)
\(x=37-14,3\)
\(x=22,7.\)