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The altitudes drawn from Q, P, R on to the opposite sides meet at Q.
So Q is the orthocenter.

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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.