can't type thta trignale thingie so subsitute with x
7*9=(7*10)-(7*x)
we can multipily what we know
7*9=7 times 9=63
7*10=7 times 10=70
now we have
63=70-(7*x)
add (7*x) to both sides
(7*x)+63=70
subtract 63 from boths ides
(7*x)=7
divide both sides by 7
x=1
the answe ris traignle is equal to 1