Answer:
a.d=20
b.f=0
c.e=-7
d.f=-1
Explanation:
A.d+d+10=50
2d+10=50
Substrate 10 from both sides
2d=40
Divide both sides by 2
d=20
B.30-f+8f=30
30+7f=30
Substrate both sides by 30
7f=0
Divide both sides by 7
f=0
C.-12=4e+23+e
Collect like terms
-12-23=4e+e
-35=5e
Divide both sides by 5
e=-7
D.42=32-6f-4f
Collect like terms
42-32=-6f-4f
10=-10f
Divide both sides by 10
F=-1