Nuclear decay
27Al + He 》 39P

Explanation:

The unbalanced nuclear equation is

We can insert the subscripts, because these are the atomic numbers of the elements

That leaves only the superscript of He to be determined,

The main point to remember in balancing nuclear equations is that the sums of the superscripts must be the same on each side of the equation.  

Then

27 + x = 31, so x = 31 - 27 = 4

Then, your nuclear equation becomes