Linear combination of regression coefficients in R [closed]

Answer 1

Note that Y = b1 * x1 + (t - b3) * x2 + b3 * x3

is equivalent to

Y = b1 * x1 + t * x2 - b3 * x2 + b3 * x3 = b1 * x1 + t * x2 + b3 * (x3 - x2)

So, you can continue from there easily.