An alpha emission is one in which an atomic nucleus emits an alpha particle (helium nucleus). Therefore in this reaction, the products of the decay are Helium and another element X. Helium has an atomic mass and atomic number of 4 and 2 respectively, therefore the complete reaction is:
U-234 (92) ---> He-4 (2) + X
Since atomic mass and number must be conserved therefore X = X-230 (90)
U-234 (92) ---> He-4 (2) + X-230 (90)
The element with atomic number of 90 is Thorium. Therefore the other product is Thorium-230 (90).
U-234 (92) ---> He-4 (2) + Th-230 (90)