Economic math

A firm produces three products—leather, cardboard and string—denoted 1,y2y and 3,y respectively. The corresponding total revenue function has the form

TR= 5 +75(y1) - 2(y1)^2 + 8(y2) - 1/2(y1)(y2) + 4(y3)^2

At what combination of outputs will the marginal revenues of all three products be simultaneously equal zero?