The most natural method for you depends on what you are learning; you haven't told us anything by which to determine that.

One elementary way is to plug the definitions of a, b, c, and d into \(\displaystyle d=\alpha a+\beta b+\gamma c\), and write a system of equations (one each for i, j, and k), then solve it for variables \(\displaystyle \alpha\), \(\displaystyle \beta\). and \(\displaystyle \gamma\).