I'm reading this example in my book and I don't get how the author gets this solution.

X1(n) = 2X1(n-1) ; x-sub-1

X2(n) = 2X2(n-1) ; x-sub-2

The solutions to these DEs, according to the book, are

X1(n) = (2^n) * X1(0)

X2(n) = (3^n) * X2(0)

I can't figure out how he makes this leap.

Thanks in advance.