One option is to use the interp1 function. Remember that if you want to find x for a given y, use (y,x) rather than (x,y).
If you are supposed to use your regression model, you want to know where the root of ‘p’ equals 120. Set the polynomial ‘p’ to have a root at 120, and use the roots function to find it. Remember that you have to subtract 120 from only the constant term of ‘p’, not the entire polynomial.