Answer:
Two wave will be coherent when they have the wavelength, amplitude, and phase of the constituent wave forms must be identical.
Where as interference is the addition of two wave considering the mathematical sense while adding them.
when two wave of same sense add together they for constructive interference. like two +ve sense or -ve sense will form constructive interference.
when the are are of different sense the form destructive interference. like one positive wave added with -ve one.