Algebraic expressions and formulas - word problem

[Mustang]

"In each formula, C represents the cost x years after 1980 of what cost $10,000 in 1975.
C=865x+15,316
C=-2x^2+900x+15,397
Use model 1 to determine the cost in 2000."

My work:
C=865(25)+ 15,316
C= 36,941

"Use model 2 to determine the cost in 2000."

My work:
C=-2x^2+900(25)+15,397
C= 36,647

Both were wrong. I subtracted 1975 from 2000 to come up with x.

[maggiemagnet]

...the cost x years after 1980....

...I subtracted 1975 from 2000 to come up with x.

Check how 'x' is defined.