How to find an expression for the expected SBP for women younger than 35 with BMI=22?

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In a study of systolic blood pressure (Y, SBP) among women, the body mass index (BMI) is recorded (taking values between 16 and 40) for each individual and is defined as r1. By default, a regression model includes an intercept. (a) [8 marks] Suppose 3 age groups are considered: younger than 35, between 35 and 65, and older than 65. Define age as a factor of 3 levels, with age between 35 and 65 as the reference level. Specify a linear regression model for SBP, with both BMI (continuous variable) and the age factor as the covariates. Interpret the coefficient for BMI in the study context. Find an expression for the expected SBP for women younger than 35 with BMI=22