Consider the matrix form of a linear regression model: Y$=$XB$+$e$..$ Show that

the least squares estimator of B is given by b$=($X$\text{'}$X$)^-1$ X$\text{'}$Y$.$ Hello, can u solve this? I dont understand how d$/$db b$\text{'}$X$\text{'}$Xb $=2$X$\text{'}$Xb$.$ Can you help?