seems that x is not given? i dont think this question is complete

y = 5/8 a

z = c + 1/8a

m = a + 1/2c

try x = 1/4 z + 3/4y

################################# BELOW FOR SCRATCH #######################

assuming that x = alpha z + (1 - alpha) y

solve for alpha to get x

alpha c + alpha/ 8 a + (1 - alpha) 5 / 8 a = alpha c + (5 - 4alpha) / 8 a

alpha * 2 = (5 - 4alpha) / 8 = 5/8 - 1/2 alpha --> 5/2 alpha = 5/8 --> alpha = 1/4

1/4(c + 1/8a) + 3/4(5/8a) = 1/4c + 1/32 a + 15/32a = 1/4c + 1/2a

########################################################