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)