a. (286 : 11 +7 x 2) x 3 - 13
= (26 + 14) x 3 - 13
= 40 x 3 - 13
= 120 - 13
= 107
b. 35 x 34 + 65 x 75 + 35 x 38 + 65 x 45
= (35 x 34) + (65 x 75) + (35 x 38) + (65 x 45)
= 1190 + 4875 + 1330 + 2925
= (1190 + 1330) + (4875 + 2925)
= 2520 + 7800
= 10 320