You can use polyfit and polyval. Here is a full demo. Don't be afraid, the actual code is only 2 lines. The rest is just code to create the variables and do some fancy plotting.
x = sort(10 * rand(1, 10));
y = 0.5 * x + 0.3 * rand(1, length(x));
% Plot the original points.
plot(x, y, 'b.', 'MarkerSize', 30);
grid on;
xlabel('x', 'FontSize',20)
ylabel('y', 'FontSize',20)
% Fit a line through the points.
coefficients = polyfit(x, y, 1); % YOU NEED THIS LINE!
% Get a fit. Find the fit values everywhere, even between the original points, just for fun.
xFit = linspace(min(x), max(x), 1000);
yFit = polyval(coefficients, xFit);
hold on;
plot(xFit, yFit, 'r-', 'LineWidth', 2)
% Find the y location where x equals exactly 6.
xDesired = 6;
xline(xDesired, 'Color', 'g', 'LineWidth',2)
yDesired = polyval(coefficients, xDesired) % YOU NEED THIS LINE!
% Draw a line from the y axis to the fit line and report what the y value
% is in the title of the graph.
line([0, xDesired], [yDesired, yDesired], 'Color', 'g', 'LineWidth', 2)
caption = sprintf('y = %f when x = %.1f', yDesired, xDesired);
title(caption, 'FontSize',20)
