`2.3^(x + 1)+4.3^(x-1)-15=51`
`2.3^(x - 1 + 2)+4.3^(x-1)=51+15`
`2.3^(x-1).3^2+4.3^(x-1)=66`
`(2.3^2).3^(x-1)+4.3^(x-1) = 66`
`(2.9).3^(x-1)+4.3^(x-1)=66`
`18.3^(x-1)+4.3^(x-1)=66`
`3^(x-1) .(18 + 4)=66`
`3^(x-1) .22=66`
`3^(x-1)=66:22`
`3^(x-1)=3`
`3^(x-1)=3^1`
`=> x-1=1`
`=>x=1+1`
`=>x=2`