Solve for one of the variables in terms of the other. If your two equations are (1) x + y = 11 and (2) 3x - y = 5, you can solve for x in the first equation by subtracting y from both sides of the equation. This yields (3) x = 11 - y.
Substitute equation (3) into equation (2). This yields 3(11 - y) - y = 5. This equation is entirely in terms of y, making it possible to solve for the numerical value of y.
Solve the modified equation. Distributing the parentheses yields 33 - 3y - y = 5, or 33 - 4y = 5. Subtracting 33 from both sides gives -4y = -28, so y must equal 7.
Substitute y = 7 back into either (1) or (2). Substituting into (1) gives x + 7 = 11, or x = 4. Thus, your answer is that x = 4 and y = 7.