Answer
$Li^7_3+H^1_1\rightarrow He^4_2+He^4_2$
Work Step by Step
First of all, we calculate the unknown atomic number as
$3+1=2+Z$
$\implies Z=2$, this shows that the element is helium He.
Now, we find the atomic mass as
$7+1=4+A$
$A=4$
Thus, we complete the given reaction as:
$Li^7_3+H^1_1\rightarrow He^4_2+He^4_2$
