How to do this can you explain it step by step

OA and OC are radii of the circle. Hence, OA = OC. So if AO = 40, then OC = 40.
Length of BO = 40 + 45
                       = 85
Using the Pythagoras theorem, you can find out the length of AB since tangents to the circle always have an angle of 90 with the circle.
AB^2 = BO^2 + AO^2
AB^2 = 85^2 + 40^2
AB^2 = 7225 + 1600
AB^2 = 8825
AB = root of 8225
AB = 90.69 to 2 decimal places.

Hopefully this helped :)