Question

construct a triangle abc with bc=8cm and angle b=45° and ab-bc=3.5cm

Answer 1

make a line BC and the make angle of 45 at b and put value of BC in ab-bc =3.5 we get
ab=11.5
$(3.5 + 8 )= 11.5$
make an arc of 11.5 with centre b of 45 and join arc to point c

Answer 2

ab-bc=3.5 => ab = 3.5 - 8 = 4.5cm  ['.' side can't be -ve]

draw bc=8cm as base. Then make a 45* angle. After that cut the 45* angle line at 4.5cm from b. join the remaining open ends.