Answer:
1. As+H2=AsH3
2. draw a line under the problem with this:
A = 1 | A = 1
H = 2 | H = 3
3. We need everything to be equal (2x3 to get 6 then count the As on the product side to get the 2, then add 2 A on the reactant side):
A = 2 | A = 2
H = 6 | H = 6
4. 2As + 3H2 = 2AsH3
hope this helps
