Maths help with equation of line and algebra?

point r is (10,0)

point s is (-9,2)

show tht length of rs is 3 root 5 not sure how to work out length of a line?

also..

x sqaured+2px+(3p+4)=0, p is a positive constant with real roots find the value of p

do i expand first?? plz help!!