Draw a line segment of any length and locate a point Q on it
Using the compasses, measure QR = 6 cm on the ruler then cut an arc on the line segment taking Q as centre. The point of intersection is R.
Construct a right angle at Q.
Cut the ray which makes right angle with QR at P with a measurement 4.5 cm
Join PR to form a right triangle.
As you know, the orthocentre of a right triangle lies at the vertex containing the right angle.
Hence, the vertex Q is the orthocentre.