As level coordinate circle geometry questions

1) find the coordinates of the points where the circle x^2+y^2-10x-8y+21=0 cuts the x axis - i know the answer.... but for some reason there two, (3,0) and (7,0), i don't have a clue how they got it.

2)a) find the equation of the tangent to the circle x^2+y^2-4x+6y-7=0

b)given that this tangent meets the x axis at point a and y axis at B find the coordaniates of A and B

3)show that y=2x+3 is a tangent to the circle x^2+y^2-12x-15y+81=0

(dont even have a clue here, i can find the equation of the tangent given the point where is crosses, but from the equation you cant work out gradient or point where it crosses)

b)find the points where the tangent touches the circle

Thankyou in advanced(Rofl)