can someone please how to do this problem.

Find y-intercept y=3.9x-4.2

I am very confused even how to start it.

Thank you

Have you not come across the slope-yintercept form of the equation??

if y = mx + b
the slope is m and the y-intercept is b

so match up your equation
y = 3.9x - 4.2 with y = mx + b