Well, there is nothing to "add" here. We can try and reduce the expression 3(63t^3)^(1/2) to something more manageable.
Whenever you have a power of a power, like (x^2)^3, you multiply the exponents. In the example the result would be x^2*3 which would be x^6. In your problem we have a fractional exponent but not to worry. Do the same operation.First lets deal with the multiplication though:
3(63t^3)^1/2 becomes (189t^3)^1/2
now the multiplication of the exponents
189t^(3*1/2) which is 189t^3/2
now we have to rewrite this expression as 189t^1 + 189t^1/2 to see if we can deal with the fractional exponent again. But there is no perfect square root of 189 so the best we can do is 189t + √189t or we could just leave it as √189t^3. but again, this has nothing to do with adding anything so I'm not sure how to answer the question ultimately.