#16: The y variables are already set to cancel. Do that, use the addition method to solve for x, then substitute the value for x into one of the equations and solve for y.
#17: Multiply the top equation by -2, then same as #16
#18: Multiply the top equation by 2 and the bottom one by -3, then the same steps as #16.
You are trying to get rid of one of the variables so you will be able to solve for one of them. Look to see what you can multiply one or both of the equations by, to get the top and bottom x or y values to equal zero when added together.
#s 25-29: Isolate a variable by subtracting, adding, multiplying, or dividing the terms to the opposite side. You want x = something, or y = something, then substitute what it equals into its spot in the OPPOSITE equation. I put some parenthesis where the x or y is and put whatever it equals inside them, then solve.