When multiplying the same base with exponents, you keep the base the same and then you add the exponents, so for this question you do:
10^3 x 10^6
10^9
This is an equation, you can't write equations as decimals. I think you're asking for x (as a decimal)?
Add 8 to each side: 4+8= 8+(-8)+3x
12=3x
x=4
This is a whole number so it's already in decimal form