Hello : the center of this cercle is : O( 0, 0) calculate the slope m of the line : OA .. A (3,-4) m= (yA - yO) / (xA- xO) m = (-4-0)/(3-0) m= -4/3 if : k the slope of the tangent : k×m = -1 (the tangent perpendiclar of : OA) k× (-4/3) = -1 k = 3/4 an equation of this tangent is : y - (-4) = (3/4)(x-3)