The answer to this question is as follows:
To get the second expression we need to multiply each term of the expression to the other term in the second expression. It is like as follows:
(ax - b)*(cx + d) =
The first term (ax) multiplies the two values of the other expression:
ax * cx + ax*d
The second term -b also multiplies the two values of the other expression:
-b*cx + (-b)*d
Then, the result of the expression is as follows:
ax * cx + ax*d -b*cx -b*d
As can you see, the pattern followed by the previous expression is the same expansion of the expression in the question. So the method is FOIL.
First ("first" terms of each binomial are multiplied together)
ax * cx
Outer ("outside" terms are multiplied—that is, the first term of the first binomial and the second term of the second)
ax*d
Inner ("inside" terms are multiplied—the second term of the first binomial and first term of the second)
-b*cx
Last ("last" terms of each binomial are multiplied)
(-b)*d
So the expression given in the question has the same pattern:
2x^2 - 1)