Y=5x-7
-3x-2y=-12
system of equations using substitution

-3x-2(5x-7)=-12
-3x-10x+14=-12

-12-14=-26
-3x-10x= -13x

So -13x=-26
x= 2

1. Since y=5x-7, then you replace the y[from the equation-3x-2y=-12] with that equation (help? Here!: y=5x-7, so -3x-2(5x-7)=-12)
2. I do not know if your teacher teach this, but make sure there is only 1 variable(like only numbers with letters: EX. 2x)
3. I still don't know if your teacher taught you this, but from the equation in the example thingy, in Step 2, move all the numbers without the variable to the other side, so the variable is left alone(example[not the real equation]: 999x=999-888, notice how the variable is alone and all the other non-variables are with other non-variables)
4. Finally divide it to find x(like you have 999x=1000-1. Solve the non-variable. So now it's 999x=999, since the equation is like 999 times x = 999, you do the opposite to find x)
5. It's complicated, really..