remember you can do ANYTHING to an equation as long as you do it to both sides
a.
3x+4=21
minus 4 both sides
3x=17
divide both sides by 3
x=17/3
b.
2x-13=3x+18
minus 2x both sides
-13=x+18
minus 18 both sides
-31=x
c. 6/15=3/x
times both sides by 15x
6x=3*15
6x=45
divide both sides by 6
x=45/6
x=15/2
a. 17/3
b. -31
c. 15/2
