Where is deltav1 equation taken from I cant find it in the b

Where is delta_v1 equation taken from? I can\'t find it in the book or anywhere in the internet.

a. Calculate the minimum total velocity increment and transit time that would be required for a two-impulse transfer from the orbit of the earth into the orbit of Mars about the sun.

Solution

According to conservation of energy ( Kinetic energy and Gravitational potential energy) are conserved