What is a material that contains three elements joined in a fixed proportiona) mixtureb) solutionc) atomd) compound

If a material contains three elements joined in a fixed proportion, it is a(an) .  

d.) compound

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer

the answer is barium and copper bromide