94
for her average to be at least 90 then letting x be the score she requires on her next test, then
[tex]\frac{85+89+92+x}{4}[/tex] = 90 ( solve for x )
multiply both sides by 4 to eliminate the fraction
85 + 89 + 92 + x = 360
266 + x = 360 ( subtract 266 from both sides )
x = 94