a,3.(5^x-1)-2=70
=>3.(5^x-1)=72
=>5^x-1=24
=>5^x=25
=>5^x=5^2
=>x=2
b,2^x+2^x+1+2^x+2=960-2^x-3
=>2^x+2^x.2+2^x.2^2=960-2^x:2^3
=>2^x.(1+2+2^2)=960-2^x:2^3
=>2^x.7=960-2^x:2^3
=>2^x.7+2^x.2^3=960
=>2^x.(7+2^3)=960
=>2^x.15=960
=>2^x=64
=>2^x=2^6
=>x=6