I tried and have no idea how to solve this problem, and need help

Question: Show that the sum of the elements in any row of H is one. (Hint: HX = X; what is the first column?)Then Show that the average of these diagonal elements is h= p

Consider a regression model in which one makes n observations on a variable Y, which varies with regressors X1, ..., Xp-1 according to Y = Bo + XIBit ...+ Xp-1Bp-1+8 In matrix terms, y = XB + & where X is the n x p matrix with columns In, Z1, ..., Zp-1 and z; contains all n values of the variables X;. Assume that the rank of X is p, so that the 'hat' matrix is H = X(XX)-1X'