The simplified version of this is -3b^2a^4.
The way that you simplify two terms like this is my multiplying the like parts. Start with the numbers in the front (don't forget that if it has a negative sign, it is like a -1.
3 * -1 = -3
b * b = b^2
a^2 * a^2 = a^4
Now we just put the parts back together,.
-3b^2a^4