Construct a 90° angle from a line p g which is 7 centimetres. From the angle byset it to get 45 centimetres.

Draw a straight line segment PQ that is 7 cm long.

Choose a point R on the line segment PQ. For simplicity, you can choose R to be at one end (e.g., P) or anywhere along the line. Let's assume you want to construct the $90^\circ$ angle at point R.

Place the compass needle at point R and draw an arc that intersects the line PQ at two points. Let's call these points A and B.

With the compass needle at point A, open the compass to a radius greater than half the distance between A and B. Draw an arc above the line PQ. Without changing the compass radius, place the needle at point B and draw another arc that intersects the first arc. Let's call the intersection point C.

Draw a straight line from point R through point C. This line will be perpendicular to PQ, forming a $90^\circ$ angle at R.

2. From the angle bisect it to get 45 degrees.

You have constructed a $90^\circ$ angle, let's say $\angle CRQ$. You need to bisect this angle.

Place the compass needle at point R (the vertex of the $90^\circ$ angle). Draw an arc that intersects both arms of the $90^\circ$ angle (RC and RQ). Let's call the intersection points D on RC and E on RQ.

With the compass needle at point D, open the compass to a radius greater than half the distance between D and E. Draw an arc inside the $90^\circ$ angle. Without changing the compass radius, place the needle at point E and draw another arc that intersects the first arc. Let's call the intersection point F.

Draw a straight line from point R through point F. This line RF bisects the $90^\circ$ angle, creating two $45^\circ$ angles ($\angle CRF$ and $\angle FRQ$).

You have now constructed a $90^\circ$ angle and then bisected it to get a $45^\circ$ angle.