4. The equation is an exponential function where all y coordinates are above 4 (range is y>4), so for y=4 (answer D) there are no solutions. y=4 is the asymptote.
5. In a formula ax^2 + bx + c the discriminant is b^2-4ac. In this example the discriminant is 9-4*3*6 = -63. The fact that this value is negative reveals that this equation has no (real) solutions, only imaginary.
6. y=2x^2-x-5. Trial and error.