Explanation:
remember the trigonometric triangle inside a circle.
sine is the up/down leg, cosine is the left/right leg.
the Hypotenuse (baseline) of the right-angled triangle is the angle-defining radius of the circle.
this is the basic definition inside the norm-circle with radius = 1.
for any other size sine and cosine need to be multiplied by the actual radius for the actual triangle side lengths.
I think you need the answer in simplified fraction and square root notification (you cut that off), but I give you also the decimal result, just in case.
so,
78 = cos(30)×y
78 = (sqrt(3)/2) × y
y = 78 / (sqrt(3)/2) = 2×78/sqrt(3) = 156/sqrt(3) =
= 90.06664199...
x = sin(30) × y = 0.5 × 156/sqrt(3) = 78/sqrt(3) =
= 45.033321...