That it asks you, is that the curve should just touch the line y = x, which is at 45 degrees with the x axis.
Can you try again with this in mind?
Find the value of the constant c so that the tangent at the origin to the curve
makes an angle of 45 degrees with the positive x axis
Not really sure what it means with the angle (tan45=1)??
I differentiated it and got c = 3x^2
Is that right?