What is the length of the red vertical edge?
brief explanation

That's a right triangle

remember, if  the legs of a right triangle measure a and b and the hypootnuse is c then a²+b²=c²

so

hypotnuse is longest side, the one oposite the right angle
in this case, it is 5
one leg is 4
and the other one is red, let's call it x

so
x²+4²=5²
x²+16=25
minus 16 both sides
x²=9
sqrt both sides
x=3

the length of the red vertical edge is 3cm