Twice a number is that number multiplied by two. So, we have
[tex] 2\cdot 2^{30} [/tex]
When you multiply powers of the same base, you sum the exponents:
[tex] a^b \cdot a^c = a^{b\cdot c} [/tex]
Which in your case means
[tex] 2^1 \cdot 2^{30} = 2^{1+30} = 2^{31} [/tex]
