Find the mass and center of mass of the lamina for density = k

where R = a triangle with vertices (0,0), (b/2,h), (b,0)

(use double integrals to solve for both the mass and center of mass!)

ive tried this problem a million times and i cant get the right answer :-( (and yeah i checked calcchat but i need a more detailed explanation....walk me through it please? :-) )