Let me just restate your question in a way that makes the answer obvious : if (x_0, y_0) and (x_1, y_1) are two of the few given points, you want to solve the system of equations y_0=a*x_0+b and y_1=a*x_1+b, where a and b are the unknown.
Let me just restate your question in a way that makes the answer obvious : if (x_0, y_0) and (x_1, y_1) are two of the few given points, you want to solve the system of equations y_0=a*x_0+by_0==a*x_0+b and y_1=a*x_1+by_1==a*x_1+b, where a and b are the unknown.