If √5 - x = 4,then x=
i need on here how to solve it

Professor Dan Healy, one of my Engineering instructors back in prehistoric times,  had a favorite way of showing us how to do this kind of problem ... I mean the kind that's incredibly easy, but your own fear gets in the way and convinces you that you can't handle it.  Here is Prof. Healy's method.

For a few moments, call the square root of 5 by the name "cow".

cow - x = 4 .    Solve this equation for 'x' :

Add 'x' to each side of the equation:  cow = 4 + x

Subtract 4 from each side:                cow - 4 = x

Now, we can put the real value back in place of 'cow' :  √5 - 4 = x

That right there is a perfect solution.  But If you absolutely need to
know the number, then grab your calculator:

√5 = 2.2361 (rounded)

x = √5 - 4 = 2.2361 - 4 = -1.7639 (rounded)