It was 40 that should be subtracted from, but mistakenly subtracting 21 from a number yields 52. How much do you get when you calculate it correctly?
52 21 [OP_ADD] 40 [OP_SUB]
var_a = 52
var_b = 21
var_c = var_a + var_b
var_d = 40
var_e = var_c - var_d
print(int(var_e))