Hello.
Because of the way you wrote the problem I am unsure if you meant y^4*y^3 or 4y*3y. So I included the solution to both.
y^4*y^3 = y^7
This is because you add exponents when multiplying them.
If you meant 4y*3y then the answer is as follows:
4y*3y = 12y
Have a nice day