Find the length of the tangent from a point 29cm away from the centre of a circle of radius 20cm

Draw a sketch and you'll see that you have a right angle triangle consisting of (a) the radius from the center of the circle to the tangent point, (b) the tangent line from that point to the point outside the circle, then (c) from that point back to the center of the circle. You know two of the three lengths - use the pythagorian theorem to solve for the third.