We try to pick out all the combinations of terms, one from each polynomial, such that their product is some number times x2.
First, if we pick 2x^3 from the first polynomial, it is impossible to get x^2 by multiplying it to another term from the other polynomial, we do not care about this case in this scenario.
Then, we consider the second term x^2 from the first polynomial. To get some multiple of x^2, we can only choose the constant term from the second polynomial, which will give us +15x2 if we multiply the terms.
Then, for -3x, we choose -11x from the second polynomial, so when we multiply them, we get +33x2
For +5, we choose tx^2 from the second polynomial, so when we multiply them we get +5tx2.
Therefore, the term in x^2 is 15x2+33x2+5tx2, which is (5t+48)x2. Since we are given that the product has no x^2 term, the coefficient must be 0, which gives t=−485.