I'll give you a hint
20) Add 56 to both sides
23) Add 1.5 to both sides
Also, as a AP Calc BC student, laziness doesn't really help get a good job soo...
#20 12=g-56
+56 +56
68=g
#23 a-1.5=4.2
+1.5 +1.5
a=5.7
