please help, I really don't understand how I work out the answer to this question and my teacher didn't teach it to us very well :/

Answer:
7x+5x+18=90
12x=90-18
12x=72
x=6
Step-by-step explanation:
pythagoras law
Answer:
Smallest angle =
[tex]7x[/tex]
[tex]7 \times 6 \\ = 42 \: \: degrees[/tex]
Step-by-step explanation:
[tex]7x + 5x + 18 = 90 \\ 12x = 90 - 18 \\ 12x = 72 \\ \frac{12x}{12} = \frac{72}{12} \\ x = 6 \\ \\ 7x \\ 7 \times 6 \\ = 42 \: \: degrees \\ \\ 5x + 18 \\ 5 \times 6 + 18 \\ 30 + 18 \\ 48 \: \: degrees \\ [/tex]