Okay. Here goes... Thanks in advance to anyone who can help.

Find the range of values ofcfor which the straight line y=3x+c intersects the curve in 2 distinct points

so this is how i did it (and it was all right, according to the answers- ill bold the part that i don't get why you do)

(because 2 distinct points)

OK, AND THIS IS THE LAST PART, BUT I HAVE NO CLUE HOW( OR WHY ITS DONE) TO GET THIS