equating the y's:-
3x + c = x^2 - 5x + 7
x^2 - 8x +7 - c = 0
If y = 3x + c is a tangent then there will be duplicate solutions to this equation
That is the discriminant b^2 - 4ac = 0. So we have:-
(-8)^2 - 4 * 1 * (7 -c) = 0
64 -28 + 4c = 0
4c = - 36
so c = -9 answer
