it'syour choiceas to what acute angle you want as theta ...

if you pick theta = 70, then

tan(70) = b/12 ... b = 12tan(70)

if you pick theta = 20, then

tan(20) = 12/b ... b = 12/tan(20)

once you have b, you can use Pythagoras to find c, or ...

sin(20) = 12/c ... c = 12/sin(20)

or ...

cos(70) = 12/c ... c = 12/cos(70)