Hello,
a = 5 * 6 - 13 = 30 - 13 = 17
b = 3 * 7 = 21
c = 5 * 4 + 5 * -7 + 5 * 2 + 5 * -6 = 20 - 35 + 10 - 30 = - 15 - 20 = -35
d = -7 * 4 - 7 * 1 + 5 * 3 = -28 - 7 + 15 = -20
a) 5 - (-7) = 5 + 7 = 12
b) 4 + 3 - 7 = 7 - 7 = 0
c) -(2 + 22) + (-14 + 52) = -24 + 38 = 14
d) - 3 - 7 - 6 - 12 = - 10 - 18 = -28