You can either solve this with the Pythagorean theorem or the special triangles rule which can be applied to (degrees —>) 45/45/90 or 30/60/90 triangles.
With the Pythagorean theorem: a^2 + b^2 = c^2 is used with the numbers accordingly- 5 and S are legs and can be either a or b while √50 being the hypotenuse can only be c
(5)^2 + (s)^2 =(√50)^2
25 + (s)^2 = 50
√ ((s)^2) = √ (50 - 25)
s = 5
The special Triangles rule actually states that a shortcut can be applied to the corresponding sides (look at picture). S is directly across from a 45 degree angle (and so is 5!)
You can use Tan(45)=S/5 to solve for S. Tan basically means Opposite/Adjacent. So the opposite side is S and the adjacent side is S. You plug tan(45) into your calc and then multiply it by 5.
Proportion Below
Tan(45)/1 = S/5
You cross multiply to get Tan(45)*5=S
S would be 5
To check you can use the Pythagorean theorem a^2+b^2=c^2