in the attachment you'll find a your slightly modified diagram.
You are dealing with a parallelogram of forces. Take the left half of the parallelogram. Since AD || BC you only have to calculate the side AD = BC. Use Sine rule:
AD/70 N = sin(45°)/sin(30°)
AD = 140*1/2*sqrt(2) N = 70*sqrt(2) N ≈ 98.994949.. N ≈ 99 N