Question 853441
Let's look at the first one.
x = 3y
2x + 3y = 15
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To solve this using substitution, take the known value of X from the first equation and put it into the second equation.
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2x + 3y = 15
2(3y) + 3y = 15 (substituted the 3y in place of the X)
Multiply
6y + 3y = 15
Add
9y = 15
Divide each side by 9
Y = {{{15/9}}}
If y = 1{{{2/3}}}, then x = 3 * 1{{{2/3}}}, or 5 [x=3y from the first equation]
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Use the same method to solve the others.  If you need further help, please post questions one at a time.