Question 228256
For this problem, you would substitute {{{x = 10 -6y}}} into {{{5x+9y=-13}}}

So, here's how it would look:

{{{5(10-6y)+9y=-13}}}

Now solve through order of operations:

{{{50-30y+9y=-13}}}

Now, combine like terms (add the y's together)

{{{50-21y=-13}}}

Next, you would get y by itself by subtracting the 50 from the left side to the right side of the equation.

{{{-21y=-13-50}}}

Then, add 13 and 50, keep the negative sign (both #'s are negative)

{{{-21y=-63}}}

Next, divide -21 onto both sides of the equation.

{{{-21y/-21=-63/-21}}}

{{{y=3}}}   
3 is positive because a negative divided by a negative is positive. 

Hope that helps.