Question

How many solution does following equtitions have 7x+21=21

Answer 1

One solution.

$7x + 21 = 21 \\ 7x = 21 - 21 \\ 7x = 0 \\ x = 0$

If we can solve for the value of x. Then the solution would be one only for system of linear equations.

For your information, When the equation is true or rather say that both sides are the same, the solution will be infinite.

When the equation is false or rather say that both sides have different number or value then there won't be any solutions.