Its a system of equations, linear with two unknowns, we'll add the equations together to eliminate one of the unknowns, then solve for one of the unknowns and finally substitute in the original equation to find the other equations.
x + 3y = 15
-8x + 3y = -12
We multiply the second equation by -1 and add this equation to the first:
x + 3y = 15
8x - 3y = 12
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9x + 0 = 27
9x = 27
x = 27/9
x = 3
then we substitute this result in the first equation given:
x + 3y = 15
3 + 3y = 15
3y = 12
y = 12/3
y = 4
so the solution is x = 3 and y = 4