Making brownies but don't have a hand whisk? Can i substitute with electric mixer?

Brownies don't need whisking. Whisks are handy tools that make it easier to blend the ingredients in together but a fork to beat the eggs and a spoon to mix everything will do just fine. You won't get any real benefit from using a mixer, you'll just make more dishes dirty.

A mixer with whisk attachment is generally a good substitute for a whisk. Both are used to integrate air into a mixture or combine the ingredients. If the recipe calls for a whisk and I have non at hand, I simply use a mixer. Overbeating shouldn't be much of a problem, if you stop as soon as everything is combined/airy enough. In case I don't have a mixer, too, or it is just a very small amount, for which a mixer would be overkill, I simply use a fork. It is basically a very rudimentary whisk. It takes longer and is definitely more tedious, however, i even made meringue with a fork once (here it helps to add a little bit of salt and have the egg whites at room temp).

That said: For brownies a good spatula or spoon should work quite fine. No mixer needed.