4. Assume the relationship between the expected return of the junk bond index and the performance of the stock market is RtHY=a+bRtSP500+c(RtSP500)2+t Where: RtHY= total return of the Bloomberg High-Yield Corporate Bond Index at time t bRtSP500= total return of the S\&P 500 Index at time t a,b,c= intercept and coefficients In the model, the intercept and coefficients are invariant over time. Please describe the regression process of estimating the model intercept and coefficients a,c and provide the analytical solution