Couple of problems. help please

hi.

can someone please help with the following:

1. The sides of a cuboid measure 1 cm, a cm and b cm. Its volume is 12cm^3 and its surface area is 38 cm^2.

Find a and b

2. Show that y=x+c is a tangent to x^2 + y^2=4 if, and only if, c^2=8. (Show that if y=x+c is a tangent then c^2=8, and also that if c^2 = 8 then y= x+c is a tangent)

thanks

with the second one ive got as far as

i know the discriminant must equal 0 and the quadratic is 2x^2 +2cx +(c^2-4)=0,

and from the first statment 4c^2-8(c^2-4)=0