It can be helpful to draw a diagram, or at least think about what such a diagram would look like. The point is in the 2nd quadrant, so the angle will be more than π/2 and less than π. You may recognize that the reference angle is 45°, and that the distance to the origin is 7√2. If not, those things can be calculated.
Number magnitude = √((real part)² +(imaginary part)²)
= √((-7)² +(7)²) = √98
Number magnitude = 7√2
Number angle = arctan((imaginary part)/(real part)) . . . . . considering quadrant
= arctan(7/-7) = arctan(-1) . . . . . in the second quadrant
Number angle = 3π/4
In polar form, -7 +7i = (7√2)∠(3π/4 radians)