the function f is continuous at t=1

if f(t) = sqrt (t +3) - sqrt (3t+1)

all divided by by (t-1) for all values of t not equal to 1, and f(t) = k when t =1, what is k?

i know you have to find the limit of f(t) as t approaches 1. but im not sure how?

THANKS! lexi