draw a circle of radius 3 cm take 2 points p and q on one of its extended diameter each at a distance of 7 cm from its center draw tangent to the circle from these two points p and q

The tangent can be constructed on the given circle as follows.
Step 1 
Taking any point O on the given plane as centre, draw a circle of 3 cm radius.
Step 2 
Take one of its diameters, PQ, and extend it on both sides. Locate two points on this diameter such that OR = OS = 7 cm
Step 3 
Bisect OR and OS. Let T and U be the mid-points of OR and OS respectively.
Step 4 
Taking T and U as its centre and with TO and UO as radius, draw two circles. These two circles will intersect the circle at point V, W, X, Y respectively. Join RV, RW, SX, and SY. These are the required tangents.

Justification
The construction can be justified by proving that RV, RW, SY, and SX are the tangents to the circle (whose centre is O and radius is 3 cm). For this, join OV, OW, OX, and OY.

∠RVO is an angle in the semi-circle. We know that angle in a semi-circle is a right angle.
∴ ∠RVO = 90°
⇒ OV ⊥ RV
Since OV is the radius of the circle, RV has to be a tangent of the circle. Similarly, OW, OX, and OY are the tangents of the circle.