The slope intercept form is:
y = mx + b
"m" is the slope, "b" is the y-intercept (the y value when x = 0)
Since you know the slope is -2, you can plug it into "m"
y = mx + b
y = -2x + b
To find "b", you can plug in the point (-3, 10) into the equation
y = -2x + b
10 = -2(-3) + b
10 = 6 + b Subtract 6 on both sides
4 = b
y = -2x + 4