We COULD solve it for you, but we WON'T. This is YOUR homework. This is the Homework Help forum, not the Homework Done For You forum.
You need to show YOUR efforts to solve YOUR homework problem. We will then use those efforts as a springboard for assisting you to find your way along a path to a solution.
To prove that a system is not linear, you should find a linear combination of two inputs a*x1(t)+b*x2(t) that does not yield a*y1(t)+b*y2(t) as an output. Try out some values of a and b, and see what happens to the output. For example, what if you take 2x as an input, do you get 2y as the output?