a)
3^{x+3}+5*3^{x-1}=86
3^x*3^3+5*3^x*3^{-1}=86
3^x(27+5*\frac{1}{3})=86
3^x(27+\frac{5}{3})=86
3^x*\frac{81+5}{3}=86
3^x*\frac{86}{3}=86/:\frac{86}{3}
3^x=\not86^1*\frac{3}{\not86^1}
3^x=3
x=1
b)
2^{x+3}+2^x=54
2^x*2^3+2^x=54
2^x(2^3+1)=54
2^x*9=54/:9
2^x=6
x=log_26
c)
7^{x+2}-2*7^{x+1}+5*7^x=280
7^x*7^2-2*7^x*7^1+5*7^x=280
7^x(7^2-14+5)=280
7^x*40=280/:40
7^x=7^1
x=1