I just need like a step by step I haven't understood what my teacher has taught me here ​

You are trying to find the side between the right angle and the degree (This is known as the adjacent)
You are given degree 42
You also given the longest side of 19 (This is known as the hypothenuse)
Cosine: adjacent/hypothenuse
Cos42 = x/19
0.7431448255 = x/19
x = 19 * 0.7431448255
x = 14.11975168, round to tenth

Solution: x = 14.1

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