Forming a triangle using the 45 degree angle both the horizontal and vertical dimensions need to be identical.
The horizontal dimension is given as 2.5m , so the vertical dimension also has to be 2.5m.
Can calculate it as:
Tan(45) = vertical/ horizontal
Tan(45) = x/2.5
Tan(45) = 1
1 = x/2.5
X = 2.5/1
X = 2.5