Give an example of an equation with an infinite number of solutions. Then make one change to the equation so that it has no solution

Two equations with infinite solutions would look the exact same. Example:

y=mx+b

y=mx+b

Example 2

y=2x+5

y=2x+5

For an equation with no solution they would have the same slope but different y intercepts. An equation with same slope and same y intercepts would have infinite solutions.

X^2 + y^2 = 1 is one equation with an infinite number of solutions.

x^2 + y^2 = - 1 has no real solutions. The only change was the sign of 1.